Apile of 40 coins consists of nickels and dimes. the total value of coins is $3.35. find the number of each type of coin

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