d = # of dimes
n = # of nickles
d + n = 40 so d = 40 - n
0.10d + 0.05n = 3.35
substitute d = 40 - n into 0.10d + 0.05n = 3.35
0.10d + 0.05n = 3.35
0.10(40 - n) + 0.05n = 3.35
4 - 0.10n + 0.05n = 3.35
- 0.05n = - 0.65
n = 13
d = 40 - 13
d = 27
answer
dimes = 27 coins and nickles = 13 coins