d=number of dimes
q=number of quarters
let's count everything in cents
dimes are worth 10 cents
quarters are worth 25 cents
10d+25q=700
divide both sides by 5
2d+5q=140
he has 7 more quarters than dimes
q=7+d
subsitute 7+d for q in other equation
2d+5q=140
2d+5(7+d)=140
2d+35+5d=140
7d+35=140
minus 35 both sides
7d=105
divide both sides by 7
d=15
sub back
q=7+d
q=7+15
q=22
22 quarters and 15 dimes