You are Here: Home » Math Tricks » Multiplication by halving and doubling

Multiplication by halving and doubling

Sometimes multiplying numbers makes you reach for a pencil and paper… Use this math to help make some bigger problems easier.

When given a problem, you can take one of the numbers and cut it in half, and then double the other number. This may result in a combination of numbers that is easier to work with. If one of your numbers is even to start with, and it continues to be even when you cut it in half, you can continue to use this trick until you get a couple of numbers that you can multiply more easily.

Let’s take a look at an example:
  14 x 8
This is not easy for me, so apply the trick – double the 14, divide the 8 by 2
  28 x 4
Getting easier, and we could probably figure it out, but let’s try the trick again
  56 x 2 = 112!
That is much easier than 14 x 8, right?

Numbers that end in 5 make great candidates for doubling, if the other number is even, because then you end up with a multiplying a number that ends in 0. Take a look:
  18 x 15 goes to 9 x 30 which equals 270!
  32 x 25 goes to 16 x 50 and then 8 x 100 which is obviously 800!

Here is an example combining a couple of math tricks:
  22 x 13 goes to 11 x 26 which then (using the 11’s Trick) equals 286!

Not all number combinations work well, but this trick is a good place to start if you need to simplify a calculation. Look for other combinations to use with this trick, and make your math a bit simpler – and more fun!