2004 AMC 12A Problem 11

If Paula has $n$ coins, their total value is $20n$ cents. Adding a quarter gives $n + 1$ coins worth $20n + 25$ cents, which must also equal $21(n + 1)$ cents. So $20n + 25 = 21(n + 1),$ giving $n = 4.$ Four coins totalling $80$ cents must be three quarters and one nickel, so the number of dimes is $0.$ Thus, the correct answer is A.