Multiply by 11

Here’s a fast mental trick for multiplying numbers by 11:

42 × 11

Take the digits of 42: 4 and 2. Add them, and the place the sum inbetween the original digits:

4 + 2 = 6
42 × 11   =   462
(This 6 goes between the digits of 4 and 2)

It's that simple!

Now, let’s try one with a carry:

93 × 11

Add the digits: 9 + 3 = 1 2
(Carry the 1, insert the 2)

9 + 1 = 10
(carry adds to the first digit so the 9 becomes a 10)
Result: 1023
(insert the 2 in between the 10 and 3 just as before)

Now try a three-digit number:

324 × 11

Add the first two numbers, then last two numbers:

3 + 2 = 5
2 + 4 = 6

Now make the sandwich:

  • Start with 3 (first digit)
  • Insert the 5
  • Insert the 6
  • End with 4 (last digit)
324 × 11   =   3564
(3564)

If any pair adds to more than 9, carry to the left just like before.

It takes some practice but is much easier once you know the technique. The fun part? Most of mental math is like that