D'anthony borrowed $ 50,000, which is the principal amount.
The repayment was $5,000 per year therefore after 15 years he will have paid a total of 15 × $ 5000 = $75,000.
Therefore, the interest accrued will be $75,000 - $ 50,000 = $ 25,000
By calculation interest is given by principal × rate×time
hence, rate = interest ×100 / principal × time
= 25,000 ×100 / 50,000 ×15
= 3.333%
Therefore, the rate of interest was 3.33% per annum