Mental Math · Addition · A3
When a number is just short of a round number, round it up, do the easy addition, then give back the little bit you over-added. This 'round and fix' move turns awkward sums into one easy step plus a tiny correction.
Watch the method
58 is close to 60. Add 60 (easy), then take back the 2 you over-added: 297 + 60 = 357, then 357 − 2 = 355.
Round 58 to the friendly number 60.
Add the friendly number — that part is easy.
You added 2 too many, so take 2 back.
Your turn
Use the method, then check. Stuck? Reveal the steps.
Adding 58 is awkward; adding 60 is easy. Round up, then subtract the extra. You are not changing the answer — you are taking a smoother road to it.
Why it works
Round and fix is just 'friendly numbers' (Foundations) used on bigger sums.
47 + 38
Round
38 → 40. Add: 47 + 40 = 87.
Fix
Added 2 too many: 87 − 2 = 85.
297 + 58
Round
58 → 60. Add: 297 + 60 = 357.
Fix
Added 2 too many: 357 − 2 = 355.
729 + 96
Round
96 → 100. Add: 729 + 100 = 829.
Fix
Added 4 too many: 829 − 4 = 825.
Rounding up but forgetting to take the extra back.
Adding 60 instead of 58 adds 2 too many. Subtract those 2 at the end.
Rounding both numbers and losing track of the fixes.
Round just one number (usually the messy one), add, then make a single correction. Keep it simple.
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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).47 + 38 = ?
65 + 28 = ?
53 + 19 = ?
86 + 9 = ?
247 + 58 = ?
364 + 29 = ?
175 + 48 = ?
432 + 19 = ?
588 + 27 = ?
347 + 88 = ?
656 + 47 = ?
729 + 96 = ?