Ahmad has a jar containing only 5-cent and 20-cent coins. In total there are 31 coins with a total value of $3.50. How many of each type of coin does Ahmad have?

Let the number of 5-cent coins be f.
Let the number of 20-cent coins be t.

We're given two equations:

f + t = 31

0.05f + 0.2t = 3.50

We can solve this simultaneous system of equations 3 ways: