In a recurring deposit account for 2 years, the total amount deposited by a person is ₹ 9600. If the interest earned by him is one-twelfth of his total deposit…

(a) Given,

Interest earned by man = One-twelfth of the total deposit

= $\frac{1}{12} \times 9600$

= ₹ 800.

Hence, the interest earned = ₹ 800.

(b) Given,

In a recurring deposit account for 2 years (or 24 months), the total amount deposited by a person is ₹ 9600.

Money deposited per month = $\frac{9600}{24}$ = ₹ 400.

Hence, monthly deposit = ₹ 400.

(c) By formula,

$\text{Interest} = \frac{P \times n \times (n + 1)}{2 \times 12} \times \frac{r}{100}$

Substituting values we get :

$$\begin{aligned}\Rightarrow 800 = \frac{400 \times 24 \times (24 + 1)}{2 \times 12} \times \frac{r}{100} \\[1em] \Rightarrow 800 = \frac{400 \times 24 \times 25}{24} \times \frac{r}{100} \\[1em] \Rightarrow 800 = 400 \times 25 \times \frac{r}{100} \\[1em] \Rightarrow 800 = 4 \times 25 \times r \\[1em] \Rightarrow 800 = 100 \times r \\[1em] \Rightarrow r = \frac{800}{100} \\[1em] \Rightarrow r = 8%.\end{aligned}$$

Hence, rate of interest = 8%.