Math 10w problem solving

Here is a really interesting question in the problem solving worksheet we got yesterday.

13. Janet has 10 coins consisting of nickels, dimes, and quarters. Seven of the coins are either dimes or quarters, and eight of the coins are either dimes or nickels. How many dimes does Janet have?

To do this question, we looked at what the question stated. Eight of the coins are dimes or nickels. What are the other two? they have to be quarters. 7 of the coins are either quarters or dimes. That means that 3 are nickels.
There are 3 nickels, 2 quarters and five other coins. The five others have to be dimes.

I was really amazed because when i found out the answer, it felt great. I found out how to use linear equations! I used to learn how to do linear equations in grade 4, but i never knew what they were for. Now I know what they are for!