Third Grade - November 10th - 14th, 2008

We had an interesting discussion with one of the homework problems last week. The challenge was to find 70 less than 231. It is always important to put a problem into a story context so that the numbers make sense. Imagine this problem as a student having 231 stickers and they give 70 away to their best friend. One way to solve this problem is by making the numbers easier to work with. Instead of having 231 stickers, imagine that you only have 230 stickers and you give away 70 of them. Now the problem becomes much more manageable. It can be thought of as 23 tens minus 7 tens which is 16 tens or 160. The question is what happened to that extra sticker. Do we have 161 or 159 stickers left over. Well, think of our story. I started with 231 stickers but I made the problem easier by changing it to 230 stickers. It is like I put the extra sticker in my pocket. After I gave away 70 of my stickers, I can pull the extra sticker out of my pocket, so now I have 161 stickers.

This week, students will look at a problem like this, but the extra sticker is being subtracted. instead of 231 - 70, think of the problem 230 - 71. Again, lets make the problem easier by solving 230 - 70 which is 160. So, what happened to the extra sticker? In this case, the extra sticker is being given away, so after I give away the 70 stickers I still have to give away one more. Instead of having 160 left over, I now have 159.

The general idea is that if 120 - 50 = 70, then 120 - 54 will be 4 less or 66. If 120 - 50 = 70, then 124 - 50 will be 4 more or 74.