Brain Exercise Through Mental Maths - Multiplying by 99

One approach to multiply a number by 99 is to first multiply it by 100 (adding two zeros at the back) and then subtracting the the number from the result. For example:  45 x 99 = 4500 - 45 = 4455.

There is even a faster way - you can almost instantaneously provide the answer as soon as the number to be multiplied by 99 is given to you.  That is the beauty of this brain exercise. You will keep discovering different and perhaps faster ways to solve the same mathematical question.

Now, the faster method is simply to minus 1 from the given number which will give you the first two digits of the answer.  The last two digits are the result of 100 minus the given number.

In our example above, you are given 45. Immediately you say 44 while your brain does the 100 - 45, which is 55.  You will be amazed, and others will too, that you are able say the answer (4455) in a flash.

I taught this to a group of Year 6 primary school students.  They caught on it without any problem and began to show off to others immediately after the class.  Their interest in mathematics also increased significantly as I taught them more 'tricks' and they discover some themselves.

Be reminded that in this brain exercise program, we will be using only 2-digit numbers for the multiplication.  Of course when we get too good at it, we will explore 3-digit numbers as well.  But that is still long ways to go.

For now, happy exercising your brain and have fun!

Next: multiplying by 11.