quarters to dimes

Question

I have eight more quarters than dimes. If the coins are worth $6.20, how many dimes are there?

Answer 1

Let D = # of dimes

Q = # of quarters = D + 8

then 

25Q + 10D = 620

25(D+8) +10D = 620

25D + 200 + 10D = 620 

35D = 420

D = 12

Q = D + 8 = 20