If Rs. 10 be allowed as true discount on a bill of Rs. 110 due at the end of a certain time, then the discount allowed on the same sum due at the end of double the time is:

Answer: Option D

S.I. on Rs. (110 - 10) for a certain time = Rs. 10.
S.I. on Rs. 100 for double the time = Rs. 20.
T.D. on Rs. 120 = Rs. (120 - 100) = Rs. 20.
$$\begin{array}{rl}& \text{T}\text{.D}\text{.}\text{on}\text{Rs}\text{.}\text{110}\\ & =Rs.\left(\frac{20}{120}\times 110\right)\\ & =Rs.18.33\end{array}$$