Step-by-step explanation:
.05N + .10D = 1.65
.05 is the value of the nickel and .10 is the value of the dime.
N + D = 21 because you have some combination of them that total 21
solve for N: N = 21- D
substitute into the first equation
0.5 (21-D) + .10D = 1.65
1.05 - .05D + .10D = 1.65
.05D = .6
D = 12 you have 12 dimes
21-12 = 9 nickels
