Mental Math · Subtraction · S3
Taking away a number close to a round one? Round it up, take away the friendly number, then add back the little bit you over-removed. It's the round-and-fix idea again — this time for subtraction.
Watch the method
38 is close to 40. Take away 40 (easy): 92 − 40 = 52. But that's 2 too many removed, so add 2 back: 52 + 2 = 54.
Round 38 to the friendly number 40.
Take away the friendly number — that part is easy.
You took away 2 too many, so add 2 back.
Your turn
Use the method, then check. Stuck? Reveal the steps.
Subtracting 40 is easier than subtracting 38. You took away 2 extra, so you return those 2. Rounding up when subtracting always means adding back.
Why it works
Take 40 dollars to be safe, then realize you only owed 38, so you keep 2.
73 − 19
Round
19 → 20. 73 − 20 = 53.
Fix
Removed 1 too many: 53 + 1 = 54.
92 − 38
Round
38 → 40. 92 − 40 = 52.
Fix
Removed 2 too many: 52 + 2 = 54.
700 − 288
Round
288 → 290. 700 − 290 = 410.
Fix
Removed 2 too many: 410 + 2 = 412.
Rounding up and then subtracting the extra (wrong direction).
When you take away too much, you must ADD back. 92 − 40 went too low, so + 2.
Mixing up the addition fix with the subtraction fix.
Adding 40 for 38 → take 2 back. Subtracting 40 for 38 → add 2 back. Picture whether you overshot up or down.
12 problems across three tiers. Auto-graded on submit. Hints and a full walkthrough on every question. Your work is saved on this device — sign in to keep it across devices.
t₁ = 5, d = 3 and t1=5,d=3 both count. For a list of numbers, separate them with commas (e.g. 3, 10, 17).92 − 38 = ?
73 − 19 = ?
85 − 29 = ?
64 − 18 = ?
245 − 58 = ?
173 − 48 = ?
326 − 29 = ?
254 − 97 = ?
532 − 196 = ?
645 − 298 = ?
700 − 288 = ?
813 − 297 = ?