A parking machine contained $3.05 made up of dimes and quarters. there were 20 coins in all. how many dimes were there?

1 dime = $0.10

1 quarter = $0.25

If we have x dimes, then we have (20-x) quarters.

x*0.1 + (20-x)*0.25 = 3.05

0.1x + 5 - 0.25x = 3.05

0.25x - 0.1x = 5 - 3.05

0.15x = 1.95

x = 1.95/0.15

x = 13 - number of dimes