Punam opened a recurring deposit account with Bank of Baroda for 1½ years. If the rate of interest is 6% per annum and the bank pays ₹11,313 on maturity, find how much Punam deposited each month?

Given,

n = 1½ years = 18 months

r = 6%

Maturity Value = ₹11,313

Let monthly deposit be P

Sum deposited = P × 18 = 18P

I = $P \times \dfrac{n(n+1)}{2 \times 12} \times \dfrac{r}{100}$

$\therefore I = P \times \dfrac{18 \times 19}{2 \times 12} \times \dfrac{6}{100}\\[1em] I= P \times \dfrac{342}{24} \times \dfrac{6}{100} \\[1em] I= P \times \dfrac{57}{4} \times \dfrac{6}{100} \\[1em] I= P \times \dfrac{342}{400}\\[1em] I = \dfrac{171}{200}P\\[1em]$

Maturity Value = Sum deposited + Interest

$Maturity Value = 18P + \dfrac{171P}{200} \\[1em] = \dfrac{3600P + 171P}{200} \\[1em] = \dfrac{3771P}{200}\\[1em] \therefore 11313 = \dfrac{3771P}{200} \\[1em] P = \dfrac{11313 \times 200}{3771} = ₹600$

Hence, Punam deposited ₹600 per month.