Mental math, made simple.
NumThink doesn't just show you tricks — it teaches you how to recognize which method fits a problem, why it works, and lets you practice it in seconds.
How it works
1 · Learn
Short, clear lessons. Why the method works, when to use it — and when not to.
Browse the methods →2 · Practice
Infinite fresh questions generated right in your browser. Instant feedback, no signup.
Start practicing →3 · Test
A 60-second sprint that measures real speed and accuracy. Chase your personal best.
Take the test →4 · Return
A new 10-question challenge every day. Same set for everyone — compare results with friends.
Daily challenge →What's different about NumThink?
Most mental-math sites teach you a pile of tricks. But the real skill is recognition: seeing 98 × 47 and instantly knowing that "round to 100 and correct" is the fast path to 4,606.
Our Strategy Trainer drills exactly that — you pick the method first, then solve. And every lesson ends in an interactive practice, because understanding without fluency fades fast.
Start with a method
How to Multiply by 5 in Your Head
To multiply by 5, multiply by 10 and then halve. 86 × 5 = 860 ÷ 2 = 430.
Level 1How to Multiply by 9 in Your Head
To multiply by 9, multiply by 10 and subtract the number once. 47 × 9 = 470 − 47 = 423.
Level 2How to Multiply by 11 in Your Head
For a 2-digit number, split it and put the digit-sum in the middle: 72 × 11 = 7_(7+2)_2 = 792.
Level 2How to Multiply by 25 in Your Head
To multiply by 25, multiply by 100 and divide by 4. 84 × 25 = 8,400 ÷ 4 = 2,100.
Level 3How to Multiply Numbers Near 100 in Your Head
Round to 100, multiply, then correct: 98 × 47 = 100 × 47 − 2 × 47 = 4,700 − 94 = 4,606.
Level 2How to Square Numbers Ending in 5
For a number ending in 5: multiply the leading part by itself-plus-one, then append 25. 85² = 8 × 9 hundreds + 25 = 7,225.