x: number of dimes
y: number of nickels
a. you have 906 coins, then:
x + y = 906
b. you have a total of $64.10, then:
0.1x + 0.05y = 64.10
c. Isolating y from the first equation,
y = 906 - x
Substituting this result into the second equation and solving for x,
0.1x + 0.05(906 - x) = 64.10
0.1x + 0.05*906 - 0.05x = 64.10
0.05x + 45.3 = 64.1
0.05x = 64.1 - 45.3
x = 18.8/0.05
x = 376
And
y = 906 - 376
y = 530
You have 376 dimes and 530 nickels