Stan has 15 coins worth $3.55. If the coins are all quarters and nickels, how many quarters and nickels does he have?

number of coins: n+q=15
amt. of money: 0.05n+0.25q=3.55
for convenience we multiply by 20:
20(0.05n+0.25q)=20(3.55)
n+5q=71
Now we subtract the coins equation from this one...
(n+5q)-(n+q)=71-15
4q=56
q=14
therefore there is 1 nickel


:)