We’ve started a blog for elementary school math practice and the theme for this week is **Up To 100**.

This theme is about pairs of numbers that add up to 100. Why this theme?

– Many money systems are based on units of 100. The dollar, for example, is 100 cents. If you buy something that costs less than a dollar, and pay for it with a dollar, the change returned to you is the number that pairs with the cost to add up to 100.

It’s important to get correct change!

– This theme is full of useful addition and subtraction facts.

– This theme builds up the idea of the “100s – complement” of a number. The 100s-complement of 70, for example, is 30. The 100s-complement of 85 is 15.

– Percentages are typically in the range 0 – 100 and it helps to understand the meaning of a percentage and its 100s-complement. A “confidence” of 95% means an uncertainty of 5%. A 15% discount means you are paying 85% of the full price.

– This theme has some surprises. It’s easy to make little errors such as “100 = 75 + 35” because 100 = 70 + 30. But the correct sum is “100 = 75 + 25”. With practice, you can create a sense for yourself of correct and incorrect. You realize that the 100s complement of 71 has to be less than 30, because 71 is more than 70.

– Later, in triangle geometry, the ideas of the “90s – complement” and “180s – supplement” are used often. So the “100s – complement” is a good foundation. You can quickly figure out 90s-complements from 100s-complements.