## Math Games: Place-value

### Complements of 10

Complements are numbers that go together in some way. In Geometry, the word complement is used to describe two angles that add up to exactly 90 degrees. For example, 70 degrees is the complement of 20 degrees.In place value, the number 90 has no special significance, but the number 10 does! The complements of ten are pairs of numbers that add up to ten: 7 complements 3, 8 complements 2, etc.When students know the complements of 10 better than any of their other sums, they are more likely to choose addition strategies that reinforce their understanding of place value. For example, when computing 7 + 6, a student might think,

"7 + 6. Well, 7 is 3 away from 10 so...
If I take the 7, plus 3 from the 6 I get 1 ten.
3 from 6 is 3.
1 ten and 3 ones is 13".

Such students honor place-value in their thinking about computation. That's a good thing!

### Collecting tens

The object of this game is to get all the dots into the two frames at the bottom of the page.
You can drag individual dots from frame to frame, or drag entire sets. Only sets of 10 can be dragged into the tens column. When all dots are accounted for, you win a check mark, and it's on to the next bunch. This simple activity is strangely addictive. Extra challenge: What's the smallest number of drags you need to finish?

### Regrouping

Sets of ten can be dragged from column to column, but single dots can not. Choose a new problem, switch from addition to subtraction, or swap the parts of a problem at any time.

Move all the dots to the bottom to complete a problem

### How many?

What numeral shows the number of dots you can see? This exercise flips the standard "how many tens are in 68?" on its ear. Students must count the tens and ones to find out which numeral is correct. Because the tens to be counted are visually distinct from the units, this game also reinforces the fact that what you count in the tens column is not what you count in the units.