Question

Ram bought a mobile for Rs.20000 at simple interest at rate of interest 15% for four equal monthly installments. And, He also bought a mixer of Rs.6000 at compound interest at rate of interest 10% for two equal monthly installments. Find the total amount paid by Ram in a month as a installment.
1. Rs. 9990.23
2. Rs. 8610.51
3. Rs. 8353.37
4. Rs. 7890.45

Answer 1

Correct Answer - Option 2 : Rs. 8610.51

Calculation:

The total price of a mobile = Rs.20000

Let monthly installment of mobile be Rs.a

Amount paid at simple interest = 20000 + 20000 × 15/100 × 4/12 = Rs.21000

⇒ 21000 = a + a × 3/12 × 15/100 + a + a × 2/12 × 15/100 + a + a × 1/12 × 15/100 + a

⇒ 21000 = 4.075a

⇒ a = 5153.37

Monthly installment for mobile = Rs.5153.37

Let monthly installment of a mixer be Rs.b

Amount paid at compound interest = 6000(1 + 10/100)² = Rs.7260

⇒ 7260 = b(1 + 10/100)^{2 - 1} + b(1 + 10/100)^{2 - 2}

⇒ 7260 = 1.1b + b

⇒ b = 3457.14

Monthly installment for mixer = Rs. 3457.14

∴ Total amount for monthly installment = 5153.37 + 3457.14 = Rs. 8610.51

Let a be the monthly installment at simple interest then,

a = P(1 + NR/1200)/ (N + N(N - 1)/2 × R/1200))