Mental Math · Foundations · F2
Round numbers like 10, 50, and 100 are easy to work with. The trick that powers a huge amount of mental math: slide an awkward number to a friendly one, do the easy calculation, then fix the small difference you created.
Watch the method
9 is awkward but 10 is friendly. Add 10 (easy!), then take 1 back because you added one too many. The total never changes — you just took an easier path to it.
Round 9 to the friendly number 10.
Add the friendly number — that part is easy.
You added 1 too many, so take 1 back.
Your turn
Use the method, then check. Stuck? Reveal the steps.
To add 9, add 10 and take 1 back. To add 19, add 20 and take 1 back. You trade a hard step for an easy step plus a tiny correction.
Why it works
Like paying with a $10 bill for a $9 item and getting $1 change — easier than counting exact coins.
If you rounded UP (added too much), take the extra back. If you rounded DOWN (added too little), add the missing bit. Always ask: did I add too much or too little?
48 + 9
Friendly
9 is almost 10. Add 10 instead: 48 + 10 = 58.
Fix
You added 1 too many, so take 1 back: 58 − 1 = 57.
67 + 19
Friendly
19 is almost 20. Add 20: 67 + 20 = 87.
Fix
You added 1 too many: 87 − 1 = 86.
297 + 8
Friendly
Round 8 up to 10: 297 + 10 = 307.
Fix
You added 2 too many: 307 − 2 = 305.
Round to a friendly number and forget to fix.
Rounding changes the value. You must undo the change: if you added 10 instead of 9, give 1 back.
Fix the wrong way (add when you should subtract).
Adding 10 for 9 means you added one TOO MANY, so you subtract 1. Picture whether you overshot or undershot.
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).48 + 9 = ?
35 + 19 = ?
76 + 9 = ?
28 + 19 = ?
123 + 9 = ?
56 + 29 = ?
234 + 19 = ?
67 + 39 = ?
297 + 8 = ?
345 + 19 = ?
88 + 39 = ?
156 + 29 = ?