Monthly Archives: October 2014

Choice of theme: Up to 100

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.