Knowledge (XXG)

Cross-multiplication

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2206: 1385:, 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 1818: 1700: 2174:
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.
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Ubiratan D'Ambrósio; Joseph W. Dauben; Karen Hunger Parshall (2014). "Mathematics Education in America in the Premodern Period". In Alexander Karp; Gert Schubring (eds.).
1395: 219: 1813:{\displaystyle x={\frac {20\ {\text{houses}}\times 100\ {\text{days}}\times 6\ {\text{builders}}}{8\ {\text{houses}}\times 10\ {\text{builders}}}}=150\ {\text{days}}.} 914: 790: 453: 993: 1695:{\displaystyle {\frac {\frac {8\ {\text{houses}}}{100\ {\text{days}}}}{6\ {\text{builders}}}}={\frac {\frac {20\ {\text{houses}}}{x}}{10\ {\text{builders}}}},} 1559:
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?
1830:" 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". 1257:
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
1549:{\displaystyle x={\frac {12\ {\text{shillings}}\times 6\ {\text{yards}}}{4\ {\text{yards}}}}=18\ {\text{shillings}}.} 1266: 1185: 813: 515: 295: 73: 1827: 2172: 1569:
in 1855, Charles Darwin declared “I have no faith in anything short of actual measurement and the Rule of Three.”
1958:"Real Decreto 126/2014, de 28 de febrero, por el que se establece el currículo básico de la Educación Primaria" 1229: 1228:
Any equation containing fractions or rational expressions can be simplified by multiplying both sides by the
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where the variable to be evaluated is in the right-hand denominator, the rule of three states that
2001: 1455:{\displaystyle {\frac {4\ {\text{yards}}}{12\ {\text{shillings}}}}={\frac {6\ {\text{yards}}}{x}}} 860:
is a variable we are interested in solving for, we can use cross-multiplication to determine that
279:{\displaystyle {\frac {a}{b}}\nwarrow {\frac {c}{d}},\quad {\frac {a}{b}}\nearrow {\frac {c}{d}}.} 2053: 1566: 1234: 194: 46: 1589:, which involved finding an unknown value where five rather than three other values are known. 977:{\displaystyle {\frac {x}{7\ {\text{hours}}}}={\frac {90\ {\text{miles}}}{3\ {\text{hours}}}}.} 2143: 2101: 2076: 2045: 1915: 1254: 509:
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
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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
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to simplify the equation or determine the value of a variable.
183:{\displaystyle ad=bc\quad {\text{or}}\quad a={\frac {bc}{d}}.} 1979:
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
1882:, though that usage is rare compared to other uses of 1714: 1605: 1478: 1398: 1318: 1269: 1188: 1145: 1072: 996: 917: 869: 816: 763: 686: 611: 518: 471: 426: 351: 298: 222: 133: 76: 2100:(Illustrated ed.). Harvard University Press. 2075:(Illustrated ed.). Harvard University Press. 1812: 1694: 1548: 1454: 1345: 1298: 1217: 1164: 1095: 1052: 976: 896: 845: 784: 736:{\displaystyle {\frac {ad}{bd}}={\frac {cb}{db}}.} 735: 666: 547: 498: 447: 398: 327: 278: 182: 105: 1465:and then using cross-multiplication to calculate 1938:. French ministry of education. 30 December 2012 1912:Handbook on the History of Mathematics Education 2014:Concise Oxford Dictionary of Quotations, 1964. 1705:which, with cross-multiplication twice, gives 1299:{\displaystyle {\frac {a}{b}}={\frac {c}{x}},} 1218:{\displaystyle {\frac {a}{1}}={\frac {x}{d}}.} 846:{\displaystyle {\frac {x}{b}}={\frac {c}{d}},} 548:{\displaystyle {\frac {a}{b}}={\frac {c}{d}},} 328:{\displaystyle {\frac {a}{b}}={\frac {c}{d}},} 106:{\displaystyle {\frac {a}{b}}={\frac {c}{d}},} 8: 2030:Studies in History and Philosophy of Science 124:are not zero, one can cross-multiply to get 1878:This was sometimes also referred to as the 1585:An extension to the rule of three was the 2026:"Charles Darwin as a statistical thinker" 1802: 1785: 1771: 1758: 1744: 1730: 1721: 1713: 1681: 1664: 1654: 1643: 1629: 1616: 1606: 1604: 1538: 1521: 1508: 1494: 1485: 1477: 1441: 1432: 1421: 1408: 1399: 1397: 1325: 1317: 1283: 1270: 1268: 1202: 1189: 1187: 1152: 1144: 1085: 1071: 1039: 1026: 1012: 1003: 995: 963: 950: 941: 930: 918: 916: 876: 868: 830: 817: 815: 762: 710: 687: 685: 651: 638: 625: 612: 610: 532: 519: 517: 478: 470: 425: 374: 352: 350: 312: 299: 297: 263: 250: 236: 223: 221: 162: 150: 132: 90: 77: 75: 2098:The Seven Pillars of Statistical Wisdom 2073:The Seven Pillars of Statistical Wisdom 1897:Brewer's Dictionary of Phrase and Fable 1871: 1592:An example of such a problem might be 1096:{\displaystyle x=210\ {\text{miles}}.} 2167:Abraham Lincoln and the Rule of Three 1136:Note that even simple equations like 7: 2096:Stigler, Stephen M (March 7, 2016). 2071:Stigler, Stephen M (March 7, 2016). 2179:- facsimile of the relevant section 1983:. Oxford: Oxford University Press. 1936:"Socle de connaissances, pilier 3" 1346:{\displaystyle x={\frac {bc}{a}}.} 897:{\displaystyle x={\frac {bc}{d}}.} 499:{\displaystyle a={\frac {bc}{d}}.} 25: 2188:The Rule Of Three in Mother Goose 1914:. Springer Science. p. 177. 2204: 2000:. London: John Hawkins. p.  1165:{\displaystyle a={\frac {x}{d}}} 417:on the right-hand side, leaving 49:, given an equation between two 1179:term is implicitly equal to 1: 249: 155: 149: 1899:. Philadelphia: Henry Altemus. 746:Cancel the common denominator 260: 233: 1: 1596:, and this can be set up as 1260:For an equation of the form 2042:10.1016/j.shpsa.2022.08.005 2248: 1132:So, 30mph×7hours=210miles. 602:= 1 on the right, getting 29: 2142:. Merriam-Webster, 1998, 987:Cross-multiplying yields 1230:least common denominator 30:Not to be confused with 2227:Fractions (mathematics) 1828:The Mad Gardener's Song 1364:of the proportion, and 580:= 1 on the left and by 67:Given an equation like 1994:Edward Cocker (1702). 1814: 1696: 1550: 1456: 1360:is referred to as the 1347: 1300: 1232:. This step is called 1219: 1166: 1097: 1054: 978: 898: 847: 786: 785:{\displaystyle ad=cb.} 737: 668: 549: 500: 449: 448:{\displaystyle ad=bc,} 400: 329: 280: 184: 107: 27:Mathematical technique 2024:Ariew, André (2022). 1815: 1697: 1551: 1457: 1348: 1301: 1220: 1167: 1098: 1055: 979: 899: 848: 787: 738: 669: 550: 501: 450: 401: 330: 281: 185: 108: 43:elementary arithmetic 2213:at Wikimedia Commons 2211:Cross-multiplication 1997:Cocker's Arithmetick 1712: 1603: 1587:double rule of three 1581:Double rule of three 1476: 1396: 1383:Cocker's Arithmetick 1316: 1267: 1186: 1143: 1070: 994: 915: 867: 814: 761: 684: 609: 516: 469: 424: 349: 296: 220: 131: 74: 55:rational expressions 18:Cross multiplication 1810: 1692: 1567:William Darwin Fox 1546: 1452: 1343: 1296: 1235:clearing fractions 1215: 1162: 1107:Alternate solution 1093: 1050: 974: 894: 843: 782: 733: 664: 545: 496: 445: 396: 325: 276: 195:Euclidean geometry 180: 103: 47:elementary algebra 41:, specifically in 2209:Media related to 1921:978-1-4614-9155-2 1805: 1801: 1791: 1788: 1784: 1774: 1770: 1761: 1757: 1747: 1743: 1733: 1729: 1687: 1684: 1680: 1673: 1667: 1663: 1649: 1646: 1642: 1635: 1632: 1628: 1619: 1615: 1541: 1537: 1527: 1524: 1520: 1511: 1507: 1497: 1493: 1450: 1444: 1440: 1427: 1424: 1420: 1411: 1407: 1356:In this context, 1338: 1291: 1278: 1210: 1197: 1160: 1088: 1084: 1045: 1042: 1038: 1029: 1025: 1015: 1011: 969: 966: 962: 953: 949: 936: 933: 929: 889: 838: 825: 728: 705: 659: 646: 633: 620: 540: 527: 491: 382: 360: 320: 307: 271: 258: 244: 231: 203:similar triangles 175: 153: 98: 85: 16:(Redirected from 2239: 2208: 2126: 2122:Sylvie and Bruno 2118: 2112: 2111: 2093: 2087: 2086: 2068: 2062: 2061: 2021: 2015: 2012: 2006: 2005: 1991: 1985: 1984: 1976: 1970: 1969: 1967: 1965: 1954: 1948: 1947: 1945: 1943: 1932: 1926: 1925: 1907: 1901: 1900: 1889:E. 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Retrieved 1952: 1942:24 September 1940:. Retrieved 1930: 1911: 1905: 1896: 1879: 1874: 1855:Turn (angle) 1822: 1704: 1593: 1591: 1586: 1584: 1571:Karl Pearson 1561: 1558: 1464: 1381: 1378: 1373: 1361: 1355: 1308: 1259: 1250: 1248: 1240: 1233: 1227: 1174: 1135: 1131: 1130: 1111: 1110: 1106: 1105: 1062: 986: 906: 855: 806: 794: 745: 676: 594: 586: 572: 564: 558:multiply by 557: 508: 457: 408: 337: 288: 212: 201:as those of 192: 115: 66: 63: 58: 36: 2165:'Dr Math', 2157:'Dr Math', 2036:: 215–223. 1884:Golden Rule 1880:golden rule 1840:Cross-ratio 39:mathematics 2232:Arithmetic 2221:Categories 1866:References 1850:Trairāśika 1845:Odds ratio 1575:Biometrika 754:, leaving 57:, one can 2058:252246047 1777:× 1750:× 1736:× 1540:shillings 1500:× 1496:shillings 1423:shillings 1018:× 797:equations 649:× 623:× 462:—getting 385:× 363:× 261:↗ 234:↖ 209:Procedure 51:fractions 2050:36113233 1891:(1898). 1834:See also 1787:builders 1760:builders 1683:builders 1645:builders 1255:Colonial 342:—we get 2150:, pp. 1362:extreme 1125:⁠ 1116:90miles 1113:⁠ 1063:and so 677:and so 599:⁠ 583:⁠ 577:⁠ 561:⁠ 2177:, 1827 2152:85-101 2146:  2104:  2079:  2056:  2048:  1964:10 May 1918:  1886:. See 1800:  1783:  1773:houses 1769:  1756:  1742:  1732:houses 1728:  1679:  1666:houses 1662:  1641:  1627:  1618:houses 1614:  1536:  1519:  1506:  1492:  1439:  1419:  1406:  1127:=30mph 1122:3hours 1083:  1037:  1024:  1010:  961:  948:  928:  856:where 199:ratios 116:where 2054:S2CID 1523:yards 1510:yards 1443:yards 1410:yards 1374:means 1087:miles 1041:hours 1028:miles 1014:hours 965:hours 952:miles 932:hours 2144:ISBN 2102:ISBN 2077:ISBN 2046:PMID 1966:2022 1944:2015 1916:ISBN 1826:'s " 1804:days 1746:days 1631:days 1368:and 1249:The 120:and 45:and 2038:doi 2002:103 1797:150 1739:100 1624:100 1080:210 803:Use 193:In 53:or 37:In 2223:: 2052:. 2044:. 2034:95 2032:. 2028:. 1895:. 1780:10 1725:20 1676:10 1659:20 1577:. 1533:18 1489:12 1469:: 1416:12 1376:. 1238:. 1021:90 945:90 752:db 750:= 748:bd 340:bd 205:. 152:or 2110:. 2085:. 2060:. 2040:: 2004:. 1968:. 1946:. 1924:. 1808:. 1794:= 1766:8 1753:6 1719:= 1716:x 1690:, 1671:x 1652:= 1638:6 1611:8 1544:. 1530:= 1516:4 1503:6 1483:= 1480:x 1467:x 1448:x 1436:6 1430:= 1403:4 1387:x 1370:c 1366:b 1358:a 1341:. 1336:a 1332:c 1329:b 1323:= 1320:x 1294:, 1289:x 1286:c 1281:= 1276:b 1273:a 1213:. 1208:d 1205:x 1200:= 1195:1 1192:a 1177:b 1158:d 1155:x 1150:= 1147:a 1119:/ 1091:. 1077:= 1074:x 1048:, 1034:3 1007:7 1001:= 998:x 972:. 958:3 939:= 925:7 921:x 892:. 887:d 883:c 880:b 874:= 871:x 858:x 841:, 836:d 833:c 828:= 823:b 820:x 780:. 777:b 774:c 771:= 768:d 765:a 731:. 725:b 722:d 717:b 714:c 708:= 702:d 699:b 694:d 691:a 662:, 657:b 654:b 644:d 641:c 636:= 631:d 628:d 618:b 615:a 595:b 591:/ 587:b 573:d 569:/ 565:d 543:, 538:d 535:c 530:= 525:b 522:a 494:. 489:d 485:c 482:b 476:= 473:a 460:d 443:, 440:c 437:b 434:= 431:d 428:a 415:d 411:b 394:. 391:d 388:b 380:d 377:c 372:= 369:d 366:b 358:b 355:a 323:, 318:d 315:c 310:= 305:b 302:a 274:. 269:d 266:c 256:b 253:a 247:, 242:d 239:c 229:b 226:a 178:. 173:d 169:c 166:b 160:= 157:a 147:c 144:b 141:= 138:d 135:a 122:d 118:b 101:, 96:d 93:c 88:= 83:b 80:a 34:. 20:)

Index

Cross multiplication
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

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