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Trade First

Closest to the traditional standard is a method that Everyday Mathematics calls

"trade first". It is the Everyday Math focus algorithm for subtraction. It is a two

stage process, first working right to left to do all the borrowing (recording the

intermediate results above the top number) and then a second pass, in any

order, doing the subtractions. The intermediate results are two-digit numbers,

so one needs to use wide columns, and it is recommended to separate the

columns with clear vertical lines. In the example at right, 325 - 58, we first

recognize that the ones column needs borrowing, so we replace 2*10+5 by

1*10+15. Then we recognize that the tens column also needs borrowing, so we

replace 3*100+1*10 by 2*100+11*10. Then we do the subtraction in each

column.

Closest to the traditional standard is a method that Everyday Mathematics calls

"trade first". It is the Everyday Math focus algorithm for subtraction. It is a two

stage process, first working right to left to do all the borrowing (recording the

intermediate results above the top number) and then a second pass, in any

order, doing the subtractions. The intermediate results are two-digit numbers,

so one needs to use wide columns, and it is recommended to separate the

columns with clear vertical lines. In the example at right, 325 - 58, we first

recognize that the ones column needs borrowing, so we replace 2*10+5 by

1*10+15. Then we recognize that the tens column also needs borrowing, so we

replace 3*100+1*10 by 2*100+11*10. Then we do the subtraction in each

column.

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Math Glossary