Mary has a total of 23 coins made up of nickels and quarters that made a sum of $3.15. How many more nickels than quarters does she have?

A) 10

B) 3

C) 8

D) 7

E) none of these

Nickels n

quarters q

I: 3.15=n*0.05+q*0.25

II: n+q=23

isolate n in II:

n+q=23

n=23-q

substitute n in I:

3.15=n*0.05+q*0.25=

3.15 = (23-q) * 0.05+q*0.25

3.15 = (23-q) * (1/20) + (q/4)

3.15 = (23/20) - q/20 + (q/4)

3.15=1.15 - (4*q/20)

2=q/5

10=q

insert q=10 into II:

n+q=23

n+10=23

n=13

so c) 3 more nickels than quarters