Think Like a Maths Genius part 2

I explained here about how to multiply any two digit number by 11 instantly.

Just to recap, you take the two numbers, add them together and then put that number between the original pair of numbers.

For example, 72 x 11

7+2 = 9

Put the 9 between 7 and 2 and the answer is: 792

72 x 11 = 792

But what do you do, when the numbers add up to a digit more than 10?

For example, 85 x 11?

8 + 5 = 13, but the answer to 85 x 11 is NOT 8135

The answer is you take your 13, and put the five in the middle as normal, but add the one to the eight.

85 x 11 = 935

Think of like this, with the 13 (i.e. the result of adding together your two numbers) in red.

1

835

935

Another example: 75 x 11

7 + 5 = 12. … Put the 2 in the middle and add the 1 to the first digit. …

75 x 11 = 825

Voila!

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