pls solve this mathematically without the use of excel. thank you.

A sinking fund is set up to accumulate $70,000. The fund pays j1 = 15%. Annual deposits of $5,000 are made to the fund. (a) How much interest does the fund earn during the year between the 5th and 6th deposits? (b) How many $5,000 deposits are required? (c) What is the size of the final smaller deposit? (Explain your answer).