cool math trick

Yesterday I spent my day at a math inservice for teachers in my district. (my last day of work... wahoo!) Part of the day was spent learning alternative algorithms for computation. To put that in plain English, we were all taught a specific way to add, subtract, multiply and divide (particularly with large numbers....think long division). Yet the way we were taught is not the only way to solve these problems... nor is it necessarily the most efficient or easiest way. It is just the way that has been taught here in America for a long time (but not necessarily in other countries.)

So we learned other ways that kids might think about problems or that might help kids be more successful at solving problems. At the very end the presenter shared a cool trick that works... but I don't really know why. I hope I can explain it. It's called the Russian Peasant Method of multiplication.

Take any two numbers to be multiplied. half the first and double the second until you reach one. (If you get a half, say 12.5, just write 12 and keep going).

For example: 24 x 25

half is 12 50 doubled

half is 6 100 doubled

half is 3 200

half is 1.5 so write 1 400 Now cross out any that start with an even number on the left (so you cross out 24, 12, and 6.) Now add the second half of the problem (the part on the right so 200 +400) This is your answer.... 600


Here's an easier number:

8 x 7

half is 4 14

half is 2 28

half is 1 56

cross out the lines that have 8, 4, and 2.... so the answer is 56

Hope that makes sense. I recognize that it isn't that fast and I don't know why it works. But it was pretty cool. Try it. You'll be surprised.