A series of $500 contributions were made at three-month intervals to a fund earning 7.5% compounded quarterly.

Question:

A series of $500 contributions were made at three-month intervals to a fund earning 7.5% compounded quarterly. The accumulated amount continued to earn 7.5% compounded quarterly for three years after the last contribution, ending the period at $13,232.56. How many $500 contributions were made?