Question: how much mnsweoney johnny had if he had 15 coins and 7 more quarters than dimes.
Let d = the number of dimes.
Let q = the number of quarters.
Then q = d + 7 (7 more quarters than dimes)
And d + q = 15 (johnny has 15 coins )
Now substitute for q = d + 7 into the 2nd equation.
d + (d + 7) = 15
2d + 7 = 15
2d = 8
d = 4
q = 11
total money is m = 4 dimes and 11 quarters = 40c + 275c = 315c = $3.15
Johnny's money is $3.15