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What sum of money is needed to invest now, so as to get Rs. 5000 at the beginning of every month forever, if the money is worth 6 % per annum compounded monthly?

Solution

Correct Option: 1

The goal is to invest a lump sum today to withdraw Rs. 5000 at the beginning of every month, forever. The interest rate is 6% per annum compounded monthly.

Since money is withdrawn at the beginning of each month (not the end), this is a perpetuity due.


The annual interest rate is 6%, compounded monthly.

Monthly Interest Rate (r) = Annual Rate ÷ 12

r=6%÷12r = 6\% ÷ 12

r=0.5%r = 0.5\% per month

In decimal form: r=0.005r = 0.005


For payments at the beginning of each period that continue forever, the present value formula is:

Present Value=Payment×(1+r)r\text{Present Value} = \frac{\text{Payment} \times (1 + r)}{r}

The (1+r)(1 + r) factor adjusts for the first payment happening immediately at month 0.


Payment = Rs. 5000

r=0.005r = 0.005

PV=5000×(1+0.005)0.005\text{PV} = \frac{5000 \times (1 + 0.005)}{0.005}

PV=5000×1.0050.005\text{PV} = \frac{5000 \times 1.005}{0.005}

PV=50250.005\text{PV} = \frac{5025}{0.005}

PV=5025×200\text{PV} = 5025 \times 200

PV=10,05,000\text{PV} = 10,05,000


Therefore, the sum of money needed to invest now is Rs. 10,05,000.

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