Multiply by 11 Instantly
Multiply any two-digit number by 11 without a single multiplication.
ab × 11 → write a, (a+b), b
1How it works
To multiply a two-digit number by 11, keep the outer digits and drop their sum in the middle. If the sum is 10 or more, carry over to the left digit.
2How to apply (in seconds)
- Split the two digits, leaving a gap between them.
- Add the two digits and place the sum in the gap.
- If the sum is 10 or more, write the units digit and carry 1 to the left.
3Worked examples
Example 1 Worked solution
Q. Find 35 × 11.
- Outer digits: 3 _ 5.
- Middle = 3 + 5 = 8.
✓ 385
Example 2 Worked solution
Q. Find 87 × 11.
- Outer digits: 8 _ 7.
- Middle = 8 + 7 = 15 → write 5, carry 1.
- 8 + 1 = 9.
✓ 957
4When to use it
- ✦Quick multiplication in data interpretation and arithmetic.
5Cautions & tips
- !Remember to carry when the middle sum reaches 10.