Subtracting TwoDigit Numbers
The sequence for subtracting twodigit 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 twodigit 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 TwoDigit Numbers