Wednesday, August 8, 2012

Multiplying More Efficiently: The Zero Trick

By 5th grade, students are working at doing math more efficiently. By this age, the expectation (and great hope) is that students have a grasp on the basic skills and can begin doing more difficult calculations, with greater speed. Speed and efficiency are important as the math gets harder, because honestly, for the kid that counts on their fingers when multiplying to do a problem like 823 x 193 would take all day.

On the very first day of school, I begin by showing the zero trick. The zero trick is very simple and, in my mind, essential for greater efficiency in multiplying.

So this zero trick, for most adults, is just common knowledge, but kids need to be taught it, and many teachers just forget about it or take for granted that it's intuitive. Just remember that nothing is intuitive in math, it needs to be explicitly taught.

I'll lay it out simply:

  • For any number ending in zero, this trick will work. 
  • If the number as a zero in it, but doesn't end in zero, this won't work.
Here we go: Let's start with an example...

500 x 32

It looks like 500 ends in two zeros. We're going to move those off to the side. That will leave us with:

5 x 32

That's a lot easier. OK, 5 x 32 = 160

Now, we have those two zeros. All we do is put them on the end. 

16000... That's the answer, it's 160 plus those two zeros on the end, for 16,000. It's as simple as that.

If both numbers end in zero, you just do it all the same. Kids are amazed when they realize that they can efficiently solve huge problems with this trick. Check it out...

5,600,000 x 1,000,000

WOW, THAT'S A LOT OF ZEROS! Let's just move them to the side and count them. 

0 0 0 0 0 0 0 0 0 0 0 = 11 zeros!!!

Now we're left with 56 x 1 = 56 (that was easy)

Now we add those 11 zeros...


And that's it! If you're not using the zero trick by 5th grade, you're wasting precious time. It's so simple, and everyone can do it.

If you'd like an easy to use worksheet, head over to Worksheet Works and build a simple one to use in class.

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