4. (20 points) A Rp 1 billion house is bought with 75% credit and 25% down payment. If the interest rate charged is j12 9% p.a. fixed for the first four years, and the loan is paid back in 15 years starting one month after it is taken, compute: a. the monthly installment b. the interest expense paid for the third year c. the principal paid for the fourth year d. the new monthly installment after the first 48 months if the interest is raised to j12 12% p.a. e. the monthly installment if the debtor from the beginning (month 1) prefers a balloon payment with Rp 200 million paid at the same time as the last installment