Subtracting Two-Digit Numbers

The sequence for subtracting two-digit numbers without regrouping is the same as that followed for addition. First, children will subtract multiples of ten, and then move to subtraction of any two-digit number, using expanded form and the subtraction algorithm. Consider the example 56 − 32.

56 — 32		50 + 6 — (30 + 2)
24		20 + 4 = 24

The expanded form illustrates why the standard algorithm of subtracting the ones and then subtracting the tens works. Parentheses are used to make the expression mathematically correct; that is, to illustrate that what is being subtracted is 3 tens and 2 ones.

Children are also shown how to use addition to check subtraction. If a, b, and c are whole numbers and c − a = b, then b + a = c.

Teaching Model 22.3: Subtract Two-Digit Numbers