Keep the Same Difference and Subtract
Add the same amount to both numbers. The difference stays the same, and the subtraction lands on a ten.
The gap between two numbers does not change if both numbers move by the same amount. 47 and 19 are 28 apart. 48 and 20 are still 28 apart. Move both numbers until the one you subtract is a ten. Subtracting a ten is removing that many tens and leaving the ones alone.
47 − 19. 19 needs 1 to become 20. Add 1 to both numbers: 47 becomes 48, and 19 becomes 20. Now subtract: 48 − 20 = 28. So 47 − 19 = 28. The picture changed, the gap did not.
- See how much the second number needs to reach the next ten.
- Add that same amount to both numbers.
- Subtract the new ten from the new larger number.
- Do not subtract that amount again. It was added to both sides.
63 − 28. 28 needs 2 to make 30, so both numbers grow by 2: 65 − 30 = 35. 52 − 17: 17 needs 3, so 55 − 20 = 35. 70 − 36: 36 needs 4, so 74 − 40 = 34.
Watch out
- Adding the extra only to the larger number. The gap grows by that extra.
- Subtracting the extra again after both numbers have already moved.
- Adding different amounts to the two numbers.
What is 63 − 28?
Add 2 to both numbers: 65 − 30 = 35.
Why can both numbers change?
They change by the same amount, so the space between them stays put. Only the subtraction gets easier.
Math practice for Same Difference
Subtract within 100 without regrouping
Practice this