Knowledge (XXG)

Cross-multiplication

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2195: 1374:, the premier textbook in the 17th century, introduces its discussion of the rule of three with the problem "If 4 yards of cloth cost 12 shillings, what will 6 yards cost at that rate?" The rule of three gives the answer to this problem directly; whereas in modern arithmetic, we would solve it by introducing a variable 1807: 1689: 2163:
Pike's System of arithmetick abridged: designed to facilitate the study of the science of numbers, comprehending the most perspicuous and accurate rules, illustrated by useful examples: to which are added appropriate questions, for the examination of scholars, and a short system of
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For example, suppose we want to know how far a car will travel in 7 hours, if we know that its speed is constant and that it already travelled 90 miles in the last 3 hours. Converting the word problem into ratios, we get
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we can multiply the terms on each side by the same number, and the terms will remain equal. Therefore, if we multiply the fraction on each side by the product of the denominators of both sides—
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In practice, the method of cross-multiplying means that we multiply the numerator of each (or one) side by the denominator of the other side, effectively crossing the terms over:
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The method is also occasionally known as the "cross your heart" method because lines resembling a heart outline can be drawn to remember which things to multiply together.
1899:
Ubiratan D'Ambrósio; Joseph W. Dauben; Karen Hunger Parshall (2014). "Mathematics Education in America in the Premodern Period". In Alexander Karp; Gert Schubring (eds.).
1384: 208: 1802:{\displaystyle x={\frac {20\ {\text{houses}}\times 100\ {\text{days}}\times 6\ {\text{builders}}}{8\ {\text{houses}}\times 10\ {\text{builders}}}}=150\ {\text{days}}.} 903: 779: 442: 982: 1684:{\displaystyle {\frac {\frac {8\ {\text{houses}}}{100\ {\text{days}}}}{6\ {\text{builders}}}}={\frac {\frac {20\ {\text{houses}}}{x}}{10\ {\text{builders}}}},} 1548:
An anonymous manuscript dated 1570 said: "Multiplication is vexation, / Division is as bad; / The Rule of three doth puzzle me, / And Practice drives me mad."
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was a historical shorthand version for a particular form of cross-multiplication that could be taught to students by rote. It was considered the height of
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This is a common procedure in mathematics, used to reduce fractions or calculate a value for a given variable in a fraction. If we have an equation
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If 6 builders can build 8 houses in 100 days, how many days would it take 10 builders to build 20 houses at the same rate?
1819:" includes the lines "He thought he saw a Garden-Door / That opened with a key: / He looked again, and found it was / A double Rule of Three". 1246:
maths education and still figures in the French national curriculum for secondary education, and in the primary education curriculum of Spain.
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Rudyard Kipling: You can work it out by Fractions or by simple Rule of Three, But the way of Tweedle-dum is not the way of Tweedle-dee.
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The mathematical justification for the method is from the following longer mathematical procedure. If we start with the basic equation
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This rule was already known to Chinese mathematicians prior to the 2nd century CE, though it was not used in Europe until much later.
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refers to his use of the rule of three in estimating the number of species in a newly discerned genus. In a letter to
1538:{\displaystyle x={\frac {12\ {\text{shillings}}\times 6\ {\text{yards}}}{4\ {\text{yards}}}}=18\ {\text{shillings}}.} 1255: 1174: 802: 504: 284: 62: 1816: 2161: 1558:
in 1855, Charles Darwin declared “I have no faith in anything short of actual measurement and the Rule of Three.”
1947:"Real Decreto 126/2014, de 28 de febrero, por el que se establece el currículo básico de la Educación Primaria" 1218: 1217:
Any equation containing fractions or rational expressions can be simplified by multiplying both sides by the
1371: 1058: 39: 2171: 788:. Cross-multiplication is a shortcut, an easily understandable procedure that can be taught to students. 1304: 855: 457: 31: 1131: 2220: 1298:
where the variable to be evaluated is in the right-hand denominator, the rule of three states that
1990: 1444:{\displaystyle {\frac {4\ {\text{yards}}}{12\ {\text{shillings}}}}={\frac {6\ {\text{yards}}}{x}}} 849:
is a variable we are interested in solving for, we can use cross-multiplication to determine that
268:{\displaystyle {\frac {a}{b}}\nwarrow {\frac {c}{d}},\quad {\frac {a}{b}}\nearrow {\frac {c}{d}}.} 2042: 1555: 1223: 183: 35: 1578:, which involved finding an unknown value where five rather than three other values are known. 966:{\displaystyle {\frac {x}{7\ {\text{hours}}}}={\frac {90\ {\text{miles}}}{3\ {\text{hours}}}}.} 2132: 2090: 2065: 2034: 1904: 1243: 498:
Another justification of cross-multiplication is as follows. Starting with the given equation
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and we can divide both sides of the equation by any of the elements—in this case we will use
2181: 2110: 2026: 1877: 749: 412: 191: 1838: 1042:{\displaystyle x={\frac {7\ {\text{hours}}\times 90\ {\text{miles}}}{3\ {\text{hours}}}},} 1984: 656:{\displaystyle {\frac {a}{b}}\times {\frac {d}{d}}={\frac {c}{d}}\times {\frac {b}{b}},} 1946: 1848: 1551: 2153: 2145: 2209: 2046: 1924: 1812: 20: 1949:. Ministerio de Educación, Cultura y Deporte. 28 February 2014. pp. 19349–19420 1843: 1559: 2176: 398:
We can reduce the fractions to lowest terms by noting that the two occurrences of
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Each step in these procedures is based on a single, fundamental property of
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The Rule of Three as applied by Michael of Rhodes in the fifteenth century
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to stand for the cost of 6 yards of cloth, writing down the equation
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Merriam-Webster's Guide to Everyday Math: A Home and Business Reference
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The Nine Chapters on the Mathematical Art: Companion and Commentary
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adopted this declaration as the motto of his newly founded journal
187: 388:{\displaystyle {\frac {a}{b}}\times bd={\frac {c}{d}}\times bd.} 50:
to simplify the equation or determine the value of a variable.
172:{\displaystyle ad=bc\quad {\text{or}}\quad a={\frac {bc}{d}}.} 1968:
Shen Kangshen; John N. Crossley; Anthony W.-C. Lun (1999).
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on the left-hand side cancel, as do the two occurrences of
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are solved using cross-multiplication, since the missing
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the same calculation can be achieved by considering the
1871:, though that usage is rare compared to other uses of 1703: 1594: 1467: 1387: 1307: 1258: 1177: 1134: 1061: 985: 906: 858: 805: 752: 675: 600: 507: 460: 415: 340: 287: 211: 122: 65: 2089:(Illustrated ed.). Harvard University Press. 2064:(Illustrated ed.). Harvard University Press. 1801: 1683: 1537: 1443: 1334: 1287: 1206: 1153: 1084: 1041: 965: 885: 834: 773: 725:{\displaystyle {\frac {ad}{bd}}={\frac {cb}{db}}.} 724: 655: 536: 487: 436: 387: 316: 267: 171: 94: 1454:and then using cross-multiplication to calculate 1927:. French ministry of education. 30 December 2012 1901:Handbook on the History of Mathematics Education 2003:Concise Oxford Dictionary of Quotations, 1964. 1694:which, with cross-multiplication twice, gives 1288:{\displaystyle {\frac {a}{b}}={\frac {c}{x}},} 1207:{\displaystyle {\frac {a}{1}}={\frac {x}{d}}.} 835:{\displaystyle {\frac {x}{b}}={\frac {c}{d}},} 537:{\displaystyle {\frac {a}{b}}={\frac {c}{d}},} 317:{\displaystyle {\frac {a}{b}}={\frac {c}{d}},} 95:{\displaystyle {\frac {a}{b}}={\frac {c}{d}},} 8: 2019:Studies in History and Philosophy of Science 113:are not zero, one can cross-multiply to get 1867:This was sometimes also referred to as the 1574:An extension to the rule of three was the 2015:"Charles Darwin as a statistical thinker" 1791: 1774: 1760: 1747: 1733: 1719: 1710: 1702: 1670: 1653: 1643: 1632: 1618: 1605: 1595: 1593: 1527: 1510: 1497: 1483: 1474: 1466: 1430: 1421: 1410: 1397: 1388: 1386: 1314: 1306: 1272: 1259: 1257: 1191: 1178: 1176: 1141: 1133: 1074: 1060: 1028: 1015: 1001: 992: 984: 952: 939: 930: 919: 907: 905: 865: 857: 819: 806: 804: 751: 699: 676: 674: 640: 627: 614: 601: 599: 521: 508: 506: 467: 459: 414: 363: 341: 339: 301: 288: 286: 252: 239: 225: 212: 210: 151: 139: 121: 79: 66: 64: 2087:The Seven Pillars of Statistical Wisdom 2062:The Seven Pillars of Statistical Wisdom 1886:Brewer's Dictionary of Phrase and Fable 1860: 1581:An example of such a problem might be 1085:{\displaystyle x=210\ {\text{miles}}.} 2156:Abraham Lincoln and the Rule of Three 1125:Note that even simple equations like 7: 2085:Stigler, Stephen M (March 7, 2016). 2060:Stigler, Stephen M (March 7, 2016). 2168:- facsimile of the relevant section 1972:. Oxford: Oxford University Press. 1925:"Socle de connaissances, pilier 3" 1335:{\displaystyle x={\frac {bc}{a}}.} 886:{\displaystyle x={\frac {bc}{d}}.} 488:{\displaystyle a={\frac {bc}{d}}.} 14: 2177:The Rule Of Three in Mother Goose 1903:. Springer Science. p. 177. 2193: 1989:. London: John Hawkins. p.  1154:{\displaystyle a={\frac {x}{d}}} 406:on the right-hand side, leaving 38:, given an equation between two 1168:term is implicitly equal to 1: 238: 144: 138: 1888:. Philadelphia: Henry Altemus. 735:Cancel the common denominator 249: 222: 1: 1585:, and this can be set up as 1249:For an equation of the form 2031:10.1016/j.shpsa.2022.08.005 2237: 1121:So, 30mph×7hours=210miles. 591:= 1 on the right, getting 18: 2131:. Merriam-Webster, 1998, 976:Cross-multiplying yields 1219:least common denominator 19:Not to be confused with 2216:Fractions (mathematics) 1817:The Mad Gardener's Song 1353:of the proportion, and 569:= 1 on the left and by 56:Given an equation like 1983:Edward Cocker (1702). 1803: 1685: 1539: 1445: 1349:is referred to as the 1336: 1289: 1221:. This step is called 1208: 1155: 1086: 1043: 967: 887: 836: 775: 774:{\displaystyle ad=cb.} 726: 657: 538: 489: 438: 437:{\displaystyle ad=bc,} 389: 318: 269: 173: 96: 16:Mathematical technique 2013:Ariew, André (2022). 1804: 1686: 1540: 1446: 1337: 1290: 1209: 1156: 1087: 1044: 968: 888: 837: 776: 727: 658: 539: 490: 439: 390: 319: 270: 174: 97: 32:elementary arithmetic 2202:at Wikimedia Commons 2200:Cross-multiplication 1986:Cocker's Arithmetick 1701: 1592: 1576:double rule of three 1570:Double rule of three 1465: 1385: 1372:Cocker's Arithmetick 1305: 1256: 1175: 1132: 1059: 983: 904: 856: 803: 750: 673: 598: 505: 458: 413: 338: 285: 209: 120: 63: 44:rational expressions 1799: 1681: 1556:William Darwin Fox 1535: 1441: 1332: 1285: 1224:clearing fractions 1204: 1151: 1096:Alternate solution 1082: 1039: 963: 883: 832: 771: 722: 653: 534: 485: 434: 385: 314: 265: 184:Euclidean geometry 169: 92: 36:elementary algebra 30:, specifically in 2198:Media related to 1910:978-1-4614-9155-2 1794: 1790: 1780: 1777: 1773: 1763: 1759: 1750: 1746: 1736: 1732: 1722: 1718: 1676: 1673: 1669: 1662: 1656: 1652: 1638: 1635: 1631: 1624: 1621: 1617: 1608: 1604: 1530: 1526: 1516: 1513: 1509: 1500: 1496: 1486: 1482: 1439: 1433: 1429: 1416: 1413: 1409: 1400: 1396: 1345:In this context, 1327: 1280: 1267: 1199: 1186: 1149: 1077: 1073: 1034: 1031: 1027: 1018: 1014: 1004: 1000: 958: 955: 951: 942: 938: 925: 922: 918: 878: 827: 814: 717: 694: 648: 635: 622: 609: 529: 516: 480: 371: 349: 309: 296: 260: 247: 233: 220: 192:similar triangles 164: 142: 87: 74: 2228: 2197: 2115: 2111:Sylvie and Bruno 2107: 2101: 2100: 2082: 2076: 2075: 2057: 2051: 2050: 2010: 2004: 2001: 1995: 1994: 1980: 1974: 1973: 1965: 1959: 1958: 1956: 1954: 1943: 1937: 1936: 1934: 1932: 1921: 1915: 1914: 1896: 1890: 1889: 1878:E. 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1856: 1853: 1852: 1851: 1849:Unitary method 1846: 1841: 1836: 1831: 1824: 1821: 1810: 1809: 1798: 1787: 1784: 1770: 1767: 1756: 1743: 1740: 1729: 1726: 1715: 1709: 1706: 1692: 1691: 1680: 1666: 1661: 1649: 1642: 1628: 1614: 1601: 1571: 1568: 1552:Charles Darwin 1546: 1545: 1534: 1523: 1520: 1506: 1493: 1490: 1479: 1473: 1470: 1452: 1451: 1438: 1426: 1420: 1406: 1393: 1343: 1342: 1331: 1326: 1322: 1319: 1313: 1310: 1296: 1295: 1284: 1279: 1276: 1271: 1266: 1263: 1235: 1232: 1215: 1214: 1203: 1198: 1195: 1190: 1185: 1182: 1162: 1161: 1148: 1145: 1140: 1137: 1093: 1092: 1081: 1070: 1067: 1064: 1050: 1049: 1038: 1024: 1011: 1008: 997: 991: 988: 974: 973: 962: 948: 935: 929: 915: 911: 894: 893: 882: 877: 873: 870: 864: 861: 843: 842: 831: 826: 823: 818: 813: 810: 793: 790: 782: 781: 770: 767: 764: 761: 758: 755: 733: 732: 721: 715: 712: 707: 704: 698: 692: 689: 684: 681: 664: 663: 652: 647: 644: 639: 634: 631: 626: 621: 618: 613: 608: 605: 545: 544: 533: 528: 525: 520: 515: 512: 496: 495: 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Retrieved 1941: 1931:24 September 1929:. Retrieved 1919: 1900: 1894: 1885: 1868: 1863: 1844:Turn (angle) 1811: 1693: 1582: 1580: 1575: 1573: 1560:Karl Pearson 1550: 1547: 1453: 1370: 1367: 1362: 1350: 1344: 1297: 1248: 1239: 1237: 1229: 1222: 1216: 1163: 1124: 1120: 1119: 1100: 1099: 1095: 1094: 1051: 975: 895: 844: 795: 783: 734: 665: 583: 575: 561: 553: 547:multiply by 546: 497: 446: 397: 326: 277: 201: 190:as those of 181: 104: 55: 52: 47: 25: 2154:'Dr Math', 2146:'Dr Math', 2025:: 215–223. 1873:Golden Rule 1869:golden rule 1829:Cross-ratio 28:mathematics 2221:Arithmetic 2210:Categories 1855:References 1839:Trairāśika 1834:Odds ratio 1564:Biometrika 743:, leaving 46:, one can 2047:252246047 1766:× 1739:× 1725:× 1529:shillings 1489:× 1485:shillings 1412:shillings 1007:× 786:equations 638:× 612:× 451:—getting 374:× 352:× 250:↗ 223:↖ 198:Procedure 40:fractions 2039:36113233 1880:(1898). 1823:See also 1776:builders 1749:builders 1672:builders 1634:builders 1244:Colonial 331:—we get 2139:, pp. 1351:extreme 1114:⁠ 1105:90miles 1102:⁠ 1052:and so 666:and so 588:⁠ 572:⁠ 566:⁠ 550:⁠ 2166:, 1827 2141:85-101 2135:  2093:  2068:  2045:  2037:  1953:10 May 1907:  1875:. See 1789:  1772:  1762:houses 1758:  1745:  1731:  1721:houses 1717:  1668:  1655:houses 1651:  1630:  1616:  1607:houses 1603:  1525:  1508:  1495:  1481:  1428:  1408:  1395:  1116:=30mph 1111:3hours 1072:  1026:  1013:  999:  950:  937:  917:  845:where 188:ratios 105:where 2043:S2CID 1512:yards 1499:yards 1432:yards 1399:yards 1363:means 1076:miles 1030:hours 1017:miles 1003:hours 954:hours 941:miles 921:hours 2133:ISBN 2091:ISBN 2066:ISBN 2035:PMID 1955:2022 1933:2015 1905:ISBN 1815:'s " 1793:days 1735:days 1620:days 1357:and 1238:The 109:and 34:and 2027:doi 1991:103 1786:150 1728:100 1613:100 1069:210 792:Use 182:In 42:or 26:In 2212:: 2041:. 2033:. 2023:95 2021:. 2017:. 1884:. 1769:10 1714:20 1665:10 1648:20 1566:. 1522:18 1478:12 1458:: 1405:12 1365:. 1227:. 1010:90 934:90 741:db 739:= 737:bd 329:bd 194:. 141:or 2099:. 2074:. 2049:. 2029:: 1993:. 1957:. 1935:. 1913:. 1797:. 1783:= 1755:8 1742:6 1708:= 1705:x 1679:, 1660:x 1641:= 1627:6 1600:8 1533:. 1519:= 1505:4 1492:6 1472:= 1469:x 1456:x 1437:x 1425:6 1419:= 1392:4 1376:x 1359:c 1355:b 1347:a 1330:. 1325:a 1321:c 1318:b 1312:= 1309:x 1283:, 1278:x 1275:c 1270:= 1265:b 1262:a 1202:. 1197:d 1194:x 1189:= 1184:1 1181:a 1166:b 1147:d 1144:x 1139:= 1136:a 1108:/ 1080:. 1066:= 1063:x 1037:, 1023:3 996:7 990:= 987:x 961:. 947:3 928:= 914:7 910:x 881:. 876:d 872:c 869:b 863:= 860:x 847:x 830:, 825:d 822:c 817:= 812:b 809:x 769:. 766:b 763:c 760:= 757:d 754:a 720:. 714:b 711:d 706:b 703:c 697:= 691:d 688:b 683:d 680:a 651:, 646:b 643:b 633:d 630:c 625:= 620:d 617:d 607:b 604:a 584:b 580:/ 576:b 562:d 558:/ 554:d 532:, 527:d 524:c 519:= 514:b 511:a 483:. 478:d 474:c 471:b 465:= 462:a 449:d 432:, 429:c 426:b 423:= 420:d 417:a 404:d 400:b 383:. 380:d 377:b 369:d 366:c 361:= 358:d 355:b 347:b 344:a 312:, 307:d 304:c 299:= 294:b 291:a 263:. 258:d 255:c 245:b 242:a 236:, 231:d 228:c 218:b 215:a 167:. 162:d 158:c 155:b 149:= 146:a 136:c 133:b 130:= 127:d 124:a 111:d 107:b 90:, 85:d 82:c 77:= 72:b 69:a 23:.

Index

Cross product
mathematics
elementary arithmetic
elementary algebra
fractions
rational expressions
Euclidean geometry
ratios
similar triangles
equations
least common denominator
clearing fractions
Colonial
Cocker's Arithmetick
Charles Darwin
William Darwin Fox
Karl Pearson
Biometrika
Lewis Carroll
The Mad Gardener's Song
Cross-ratio
Odds ratio
Trairāśika
Turn (angle)
Unitary method
Golden Rule
E. Cobham Brewer
"Golden Rule"
ISBN
978-1-4614-9155-2

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