Mental Math · Friendly Numbers
Bend any add-or-subtract problem into a friendly number and finish it in your head before your pencil even moves.
1 Explore — try these first
Try before you watch. Pick a level below and give the problem an honest try on paper first — wrong turns and all. Then open the video to see the trick. Every level rides one habit: Bend a number to a friendly base, keep the change — the answer stays the same, the math gets easy.
★ · Bend and Repay
You see sums like 9 + 6, 19 + 7, and 98 + 5. What does 9 + 6 equal?
L0 · Make Ten
You add 8 + 5. What does 8 + 5 equal?
L1 · Make Hundred (Closest-Sum Cards)
You get four number cards: 2, 4, 6, and 7. Use each card once to build two 2-digit numbers. Make their sum land as close to 100 as you can — what is the closest sum you can reach?
L2 · Round and Repay (2-digit)
You add 54 + 38. What does 54 + 38 equal?
L3 · Subtract by Rounding the Subtrahend Up
You work out 63 − 28. What does 63 − 28 equal?
L4 · Make Thousand (Hundreds-First)
You get six number cards: 1, 2, 3, 5, 6, and 8. Use each card once to build two 3-digit numbers. Make their sum land as close to 1000 as you can — what is the closest sum you can reach?
L5 · Safe-Side Rounding
Three things cost $19, $22, and $38, and you have a $90 bill. Round each price up: will $90 be enough?
2 Learn — watch the solutions
Gave each one a real try? Now watch the trick. (Stuck is fine — that's the point.)
★ · Bend and Repay
Peek the trick — Bend and Repay
If a number is close to a friendly base, bend it up to the base, do the easy math, then repay exactly what you nudged. The answer never changes — only the work gets friendlier.
L0 · Make Ten
Peek the trick — Make Ten
When a single-digit sum crosses ten, fill the first number up to ten using part of the second, then add whatever is left over. Ten is always your friend.
L1 · Make Hundred (Closest-Sum Cards)
Peek the trick — Make Hundred
When you build two 2-digit numbers from given digit cards to land near 100, aim the columns: make the tens add to 9 and the ones add to 10, so the ones carry one ten and 90 + 10 = 100.
L2 · Round and Repay (2-digit)
Peek the trick — Round and Repay
If an addend is just under a ten, round it up to that ten, add the easy way, then repay what you added. It is Bend and Repay with bigger numbers.
L3 · Subtract by Rounding the Subtrahend Up
Peek the trick — Subtract, Round Up
When you take away a number just under a ten, round THAT number up to the ten and subtract — but because you removed too much, ADD BACK what you over-removed. It is the opposite of addition's repay.
L4 · Make Thousand (Hundreds-First)
Peek the trick — Make Thousand
To build two 3-digit numbers from digit cards aiming at 1000, work from the left: make the hundreds add to 9, the tens add to 9, and the ones add to 10. The carry chain gives 900 + 90 + 10 = 1000. When the cards cannot line up, take the closest.
L5 · Safe-Side Rounding
Peek the trick — Safe-Side Rounding
When an estimate must not fall short — enough money, enough time, enough seats — round every quantity UP so your estimate stays safely on the high side. If even the rounded-up total fits, you are safely covered.
3 Master — practice on your own
Print the practice sheet and solve without the videos. Check your answers at the back — if one is wrong, the answer key names the trick so you know exactly which video to rewatch.
Download fresh practice problems PDF
