Explanation:
d = number of dimes
n = number of nickels
one dime = $0.10
one nickel = $0.05
d = 3n
0.1×d + 0.05×n = 3.85
0.1×3n + 0.05×n = 3.85
0.3×n + 0.05×n = 3.85
0.35×n = 3.85
n = 3.85 / 0.35 = 11
d = 3n = 3×11 = 33
she has 33 dimes (and 11 nickels).