Apile of 55 coins, consisting of nickels and dimes, is worth $3.90. find the number of each.

Answer is: 32 nickels and 23 dimes.
x - number of nickels .
y -number of dimes.
two equations in two unknowns:
1) x + y = 55
2) 0,05x + 0,1y = 3,90.
nickel is a five-cent coin.
dime is a ten-cent coin.
solve equation 1) for x and substitute for x in equation 2)
x = 55 - y
0,05·(55 - y) + 0,1y = 3,902,75 - 0,05y + 0,1y = 3,90
0,05y = 1,15
y = 23.
x = 55 - 23 = 32.