AMC 12 2004a Problem 11

The average value of all the pennies, nickels, dimes, and quarters in Paula's purse is $20$ cents. If she had one more quarter, the average value would be $21$ cents. How many dimes does she have in her purse?

$\text {(A) }0 \text {(B) } 1 \text {(C) } 2 \text {(D) } 3 \text {(E) }4$