A problem about coins

I’ve started to have my younger son work through a few old AMC 8 problems each day to get a little variety in the math he’s seeing. It is hard to come up with good pre-algebra problems, so I really appreciate the quality of these old AMC problems.

It is also always interesting to me to see the problems that give the kids a little trouble. Today a problem about coins tripped up my son a bit, but we ended up having a great discussion about it.

Here’s the problem:

2010 AMC 8 problem number 7

and here’s our two part discussion. Part of the trouble my son had came from looking at the complicated examples first. He starts looking at easier examples in the middle of the second video and things click fairly quickly. Fun problem and a fun discussion.