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The 11’s trick

A cool math trick to help you multiply many (but not all…) two digit numbers by 11 – very quickly and in your head!

Let’s start with a simple example:

  11 * 12 = 132

Do you notice anything about the product? (The “product” is what the answer of a multiplication problem is called)

See how the answer 132 has the 1 and 2 digits from the 12? And, the 1 and 2 add up to 3, which is also in the answer!

This same pattern of having the original two digit number and the sum of those two numbers in the middle can be used with other two digit numbers multiplied by 11.

So, to quickly multiply a two digit number by 11, split the first and second digit, add them together and put that sum in between them. This trick only works for two digit numbers that add up to 9 or less, but that is a big set of numbers that you can now multiply by 11 very quickly!

Another example :

  45 * 11= 495 split the 4 and 5, add them, put the 9 between them and – yahoo! Victory!

Now try these – how fast can you get them?

  62 * 11 =
  25 * 11 =
  11 * 17 =
  11 * 54 =