A sum of Rs. 10 is lent to be returned in 11 monthly instalments of Rs. 1 each, interest being simple. The rate of interest is:

Answer: Option D

⇒ Rs. 10 + S.I. on Rs. 10 for 11 months
= Rs. 11 + S.I. on Rs. 1 for (1 + 2 + 3 + 4 + ........... + 10) months
⇒ Rs. 10 + S.I. on Rs. 1 for 110 months
= Rs. 11 + S.I. on Rs. 1 for 55 months
S.I. on Rs. 1 for 55 months = Rs. 1
$$\begin{array}{rl}& \therefore \text{Rate}=\left(\frac{100\times 12}{1\times 55}\right)\%\\ & =21\frac{9}{11}\%\end{array}$$