Use This Strategy to Easily Multiply Large Numbers and Fractions

You know the guys who easily multiply in their head who leave you picking your jaw up off of the floor?  Well, these folks have special strategies.  I am going to teach you one of these so that you can teach your students!

First, let’s look at this example with fractions.  If you double ½ you get one.  Instead of going through all of the steps it takes to multiply fractions, why not simply double the fraction and multiply?  In the case of one half or any other fraction with an even denominator, this process is simple.  ½ becomes 1.  Then multiplying by 1 is super simple.

In the case of a denominator such as ¼,  in the second example, you can double the number twice and halve the other number until you find a factor that is easy to multiply.  ¼ doubled becomes ½ and ½ doubled becomes 1.  As long as the other factor is easy to halve, this works great!

This may be done with mixed numbers as well.  As long as one of the numbers is even, you can double the other.

Now let’s look at examples with whole numbers.  Again, double one factor and halve the other.  Hmmmm 6 x 24. I don’t know that in my head, but I do know that I can easily double 6 to 12 and halve 24 to 12.  Wow!  I do know 12 x 12! 144!

I will skip the next two examples (12 x15 and 25 x 16) because these are self-explanatory.

Let’s look at 6 x 32.  If we double 6 and halve 32, we get 12 x 16.  Still not an easy fact.  Ok, I will try to double and halve again, and I get 24 x 8.  Hmm, again I don’t know that one.  Let’s try another time.  We get 48 x 4.  Whew!  Still difficult.  One more time.   Ok, 96 x 2.  To solve this problem, I will use a combination of strategies.  First I know that 96 is 4 away from 100.  If I have two groups of 4 away from 100, then I know that I will be 8 away from 200 because 96 is almost 100.  If I take 8 away from 200, this gives me 192.  Teach children through number talks etc. to think flexibly about numbers and ways to solve problems.  By teaching children these strategies, you will become stronger at solving math problems in your head as well!