A bag contains 77 coins consisting of quarters and nickels. The total value of the coins is $12.50. Write a system of equations that can be used to determine the number of quarters, x, and the number of nickels, y, in the bag. a
x + y = 77
0.25x + 0.5y = 12.5

b
0.25x +0.5 y = 77
x + y = 12.5

c
x + y = 77
0.25x + 0.05y = 12.5

d
0.25x + 0.05y =77
x + y = 12.5