I cleaned out my piggy bank and found 15 coins made up of dimes and quarters. How many dimes did I have?

Solution:

I have x dimes and (15-x) quarters

I have A dollars in total, A is a constant

0.1x+0.25(15-x) = A

x = (3.75-A)/0.15

So I have (3.75-A)/0.15 dimes