When working problems of this sort, it is usually convenient to let the variable represent the highest-value contributor. Here, we choose to let q represent the number of quarters. Then the number of dimes is (29 -q) and the total value is
25q +10(29 -q) = 560 . . . . . . . . . . value in cents
15q = 560 -290
q = 270/15 = 18
As we said, the number of dimes is 29 -q = 11.
I have 18 quarters and 11 dimes.