Tanvy has a recurring deposit account in a finance company for 1½ years at 9% per annum. If she gets ₹15,426 at the time of maturity, how much per month has been invested by her?

Given,

T = 1½ years = 18 months

r = 9%

Maturity Value = ₹15,426

Let monthly deposit be P

Sum deposited = P × 18 = 18P

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

$\therefore I = P \times \dfrac{18 \times 19}{2 \times 12} \times \dfrac{9}{100}\\[1em] = P \times \dfrac{342}{24} \times \dfrac{9}{100} \\[1em] = P \times \dfrac{57}{4} \times \dfrac{9}{100} \\[1em] = P \times \dfrac{513}{400}\\[1em] \text{Maturity Value} = 18P + \dfrac{513P}{400} \\[1em] = \dfrac{7200P + 513P}{400} \\[1em] = \dfrac{7713P}{400}\\[1em] \therefore 15426 = \dfrac{7713P}{400} \\[1em] P = \dfrac{15426 \times 400}{7713} = ₹800$

Hence, Tanvy deposited ₹800 per month.