Box 6–9 Three Methods for Multidigit Subtraction
A Common U.S. Algorithm
Accessible Generalizable Methods
Do all ungrouping, in any order, until every top number is larger than the bottom number. Then subtract each kind of multiunit, in any order.
the ones place to get 6, a student moves to the left and sees 3 on the top and 6 on the bottom. The answer 3 is generated spontaneously as a subtraction answer, given 6 and 3. It takes extra effort to suppress this answer and think about the direction in which one is subtracting.
Methods B and C are slight variations of Method A in which Step 1 (regrouping) is done for all columns first. For each column in either direction, the student asks the regrouping (borrowing) question, “Can I subtract in this column? Is the top digit as big as or bigger than the bottom digit?”