Subtraction with Borrowing

This unit builds the habit of breaking a group of ten apart in order to subtract. For 13 - 7, the 13 is split into 10 and 3; subtracting 7 from 10 leaves 3, which combines with the remaining 3 to give 6. Calculation, Fill in the Blank and Word Problems all lead toward single-digit answers.

The classic error is reversing the ones digits so they can be subtracted the easy way. Writing 4 for 13 - 7, from the gap between 3 and 7 alone, means the child never started breaking the ten apart. Answering only the 3 from 10 - 7 and forgetting to add the other 3 is also common.

Grouping numbers into a ten is already familiar from Addition with Carrying. Here that idea runs in reverse: a ten is broken apart so that subtraction can proceed. Only the direction of splitting the number changes, and the same reasoning is picked up again for borrowing in 2-Digit Subtraction in grade 2.