A mortgage loan is repaid with annual installment payments payable at the end of each year for 30 years. Each subsequent payment is 2% higher than the previous one. The interest rate charged on the loan balance is:
- 10% in the years 6k + 1,
- 9% in the years 6k + 2,
- 7% in the years 6k + 3,
- 8% in the years 6k + 4,
- 7.5% in the years 6k + 5,
- 8.5% in the years 6k + 6,
Where k = 0, 1, 2, 3, 4. The principal repaid in the 28-th installment is 33,990. Find the original amount of the mortgage loan.