Nick has some quarters and dimes. He has 17 coins worth a total of $3.35. How many of each type of coin does he have?

Equations:
q + d = 17
0.25q + 0.10d = 3.35$
———————
-0.10( q + d = 17)
-0.10q -0.10d = -1.7

-0.10q -0.10d = -1.7
0.25q + 0.10d = 3.35
Add the equations together
0.15q = 1.65
/0.15 on both sides
q = 11
———————
q + d = 17
11 + d = 17
-11 on both sides
d = 6
———————
0.25q + 0.10d = 3.35
0.25(11) + 0.10(6) = 3.35
2.75 + 0.60 = 3.35
3.35 = 3.35
———————

Answers:
11 Quarters
6 Dimes