Mental Math Tricks in Action: Multiplying Numbers Between 90-99

By | Added Dec 3, 2014

At Mathnasium we constantly stress the importance of mental math and numerical fluency to our students and parents. Not only is it incredibly useful to be able to do mental math on the spot, it's cool too! 

Need proof? Just check out this video of Houston Texans quarterback Ryan Fitzpatrick's 6 year old son Brady stealing the show from his father at a press conference with an impressive display of mental multiplication!

 


So how can YOU impress your friends and family by quickly multiplying numbers from 90-99? We'll show you the trick!

Let's walk through Brady's problem: 

93 x 97 = ???? 


Step One: Find the first two digits of the answer. 

????

To do this, subtract each number from 100. 

100 - 93 = 7 

100 - 97 = 3 


Now take the largest of the two answers, and subtract it from the largest of the two numbers you are multiplying. 

100 - 93 = 7

100 - 97 = 3 

In this case, 97 is greater than 93, and 7 is greater than 3, so we'll use those.

97 - 7 = 90

Voila! We have our first two digits = 90.

 
(Side note: You can also subtract the smallest of the two [100-n] answers from the smallest number you are multiplying and get the same answer. In this case, we would use 93 - 3 = 90).


Step Two: Find the last two digits of the answer.

90??

 

Now we use the two answers we found before by subtracting from 100, and we multiply them together.

100 - 93 = 

100 - 97 = 

7 x 3 = 21

Now we have our last two digits = 21.

So our final answer is 93 x 97 = 9021. 

In the words of Ryan Fitzpatrick, "BOOM!"

 

Try this method for yourself!  See if you can solve this problem:

96 x 93 = ????

(Scroll down for the answer).
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Answer: 8928
 

First we find our first two digits: 

????

100 - 96 = 4

100 - 93 = 7 

96 - 7 = 89

Next we find our second two digits:

89??

100 - 96 = 4

100 - 93 = 

4 x 7 = 28

Voila! 8928 is our answer.