Mental multiplication
Six methods cover an enormous share of the mental multiplication you'll ever need. Learn them in order — each builds on the one before — then pressure-test yourself with the timed test.
How to Multiply by 5 in Your Head
To multiply by 5, multiply by 10 and then halve. 86 × 5 = 860 ÷ 2 = 430.
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Method 2 · 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.
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Method 3 · 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.
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Method 4 · 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.
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Method 5 · 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.
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Method 6 · 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.
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