Keep the outside. Add the middle.
Multiply by 11 in your head
An original website mini lesson · Learning On My Own
The idea
For a two-digit number, keep the tens digit on the left and the ones digit on the right. Add the two digits to find the middle.
Worked example
43 × 11
- Keep 4 on the left and 3 on the right.
- Add them: 4 + 3 = 7.
- Put 7 in the middle: 473.
Why it works: 43 × 11 = 430 + 43 = 473.
When the middle needs two digits
Try 78 × 11. The sum is 7 + 8 = 15. Put the 5 in the middle and carry the 1 to the hundreds digit: 7 + 1 = 8. The answer is 858.
Check it: 780 + 78 = 858. Don't put all of “15” between 7 and 8.
Your turn
Find 64 × 11 without a calculator.
See the answer and steps
6 + 4 = 10. Put 0 in the middle, carry 1 to the left: 6 + 1 = 7. So the answer is 704. Check: 640 + 64 = 704.
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