This Week's Mental Math Moment - Letting Go of the Calculator

Multiplying Two Digit Numbers

Many of us find it relatively easy to multiply by 2, 3, 5, and 10.  If we can express our two-digit numbers as multiples of some of these numbers, then it will make  multiplying them much easier.

Let’s start with a simple example:  12 x 18.  We have all learned our multiplication tables up to 12, so let’s factor  the 18 into 9 x 2. 

Now we have the following:

            12 x 18 = 12 x 9 x 2

                       = 108 x 2

                       = 216

Another example is 24 x 21.  Factoring 24 by 2 and 21 by 3, we can multiply as follows:

           24 x 21 = 12 x 2 x 7 x 3

                      = 12 x 7 x 3 x 2  (Using the Commutative Property)

                      = 84 x 3 x 2

                      = 252 x 2

                      = 504

This takes some practice, but you’ll be amazed by what you can do if you give it a try!