The answer is
n = 37 - d
0.05n + 0.10d = 3.10
He has 25 dimes.
n - the number of nickles
d - the number of dimes
n = $0.05
d = $0.10
Carter has 37 coins: n + d = 37
The value of the coins is $3.10: 0.05n + 0.10d = 3.10
n + d = 37
0.05n + 0.10d = 3.10
_____
n = 37 - d
0.05n + 0.10d = 3.10
_____
0.05(37 - d) + 0.10d = 3.10
1.85 - 0.05d + 0.10d = 3.10
1.85 + 0.05d = 3.10
0.05d = 3.10 - 1.85
0.05d = 1.25
d = 1.25 : 0.05
d = 25