Fast ways to add and subtract

Mathematics · Addition & Subtraction · ages 8-9

Name ______________________ Date ____________

Only one of these equations is true. Which one?
- 45 + 23 = 98 - 30
- 36 + 40 = 91 - 14
- 27 + 30 = 81 - 23
- 52 + 14 = 90 - 25
Which is the fastest way to add 297 + 145?
- Round 297 up to 300, add 145, then subtract 3
- Add the ones column only
- Start counting from 1 up to 297
You need to add 500 + 287. What makes this problem easy?
- 500 is a round number, so you can add 287 to it directly, with no regrouping
- Both numbers have the same number of digits
- 287 is an odd number
To subtract 634 − 275, you may need to regroup a ten and a hundred.
Circle one: True False
Which addition problem is easiest to solve by rounding, because one number is very close to a hundred?
- 256 + 371
- 418 + 263
- 199 + 342
What is 503 - 216?
- 287
- 297
- 387
- 313
What is 356 + 478?
Answer: ______________
Use rounding and adjusting to add 396 + 258. Type the total.
Answer: ______________
Use each of the digits 1 to 9 at most one time. Which subtraction has the greatest difference?
- 987 - 123
- 965 - 432
- 789 - 321
- 912 - 876
Priya subtracted 723 − 465 and got 342. She subtracted each column by taking the smaller digit from the bigger digit, ignoring which number was on top. What's the fix, and what is the correct answer?
- She needed to add instead of subtract
- She should have regrouped instead of flipping digits; the correct answer is 258
- Her answer is correct, since subtraction can go either way in a column

Answer key

For grown-ups. Fold this page away before handing over the rest.

45 + 23 = 98 - 30 · Both sides of the first equation equal 68. Checking both sides is a fast way to find the true equation.
Round 297 up to 300, add 145, then subtract 3 · 297 is close to 300, so rounding up and adjusting back is quicker than counting or the written method.
500 is a round number, so you can add 287 to it directly, with no regrouping · Adding to a round hundred like 500 is quick because there's nothing to carry.
True · The ones and tens in 634 are smaller than in 275, so you regroup twice to subtract.
199 + 342 · 199 is just 1 away from 200, so rounding and adjusting is the fastest strategy here.
287 · Regroup across the zero first, then subtract each place. 503 - 216 = 287.
834 · Add the ones, then the tens, then the hundreds, carrying extra tens and hundreds as you go. 356 + 478 = 834.
654 · Round 396 up to 400 (add 4), get 400 + 258 = 658, then subtract the 4 back: 654.
987 - 123 · To make the greatest difference, make the top number as big as possible and the bottom number as small as possible. 987 - 123 = 864, which is the biggest here.
She should have regrouped instead of flipping digits; the correct answer is 258 · When the top digit is smaller, you regroup from the next place instead of subtracting it backwards. The correct answer is 258.