Rekha opened a recurring deposit account for 20 months. The rate of interest is 9% per annum and Rekha receives ₹441 as interest at the time of maturity. Find the amount Rekha deposited each month.

Here,
n = number of months for which the money is deposited = 20,
r = interest rate per annum = 9

Let the monthly installment be ₹x, then P = ₹x.

Using the formula:

$I = P \times \dfrac{n(n+1)}{2 \times 12} \times \dfrac{r}{100} \text{, we get} \\[0.7em] I = \Big( x \times \dfrac{20 \times 21}{2 \times 12} \times \dfrac{9}{100} \Big) \\[0.5em] \enspace\medspace = ₹1.575x$

According to the given,

$1.575x = 441 \\[0.5em] \Rightarrow x = \dfrac{441}{1.575} \\[0.5em] \Rightarrow x = 280 \\[0.5em]$

∴ The monthly installment = ₹280