February 2, 2010

photo by woodleywonderworks

The idea that you simply add a zero to a number when you multiply it by 10 is probably one you already know, but it’s a fun one to teach beginners.

After that, however, did you stop to think that just as 5 is half of 10, when you multiply a number by 5, it is half of what it is when it’s multiplied by 10?

Here’s what I mean – something like this:

• 5 x 5 . . . I know 5 x 10 is just 5 plus a zero on the end = 50, so 5 x 5 is half of 50 = 25.
• 7 x 5 = half of 70 = 35.
• 16 x 5 = half of 160 = 80

Or you can think of it this way – divide the multiplicand in half and then times 10:

• 5 x 5 = half of 5 is 2.5 . . .2.5 x 10 = 25
• 7 x 5 = half of 7 is 3.5 . . . 3.5 x 10 = 35
• 16 x 5 = half of 16 is 8 . . . 8 x 10 = 80

It is an extra step, but it’s definitely faster than counting by 5s on your fingers to get to the answer.  Plus, it’s a big help once you move up into the bigger numbers you don’t have memorized.  It also help teach relationships between numbers, another fun trick.

Enjoy!

P.S.  Here are some previous tips and math reviews:

• Tips and Tricks for 9′s
• Tips and Tricks for 6′s
• Timez Attack
• Dojo Math
• Favorite Math Curriculum
• Memorize in Minutes: the Times Tables

Posted under Math Curriculum