In a library the ratio of story books and other books is 7 : 2 and there are 1512 story books. Due to collection of some more story books the ratio becomes 15 : 4. The number of story books collected is = ?

Answer: Option A

According to the question,
$$\begin{array}{rl}& \frac{\text{Story books}}{\text{Other books}}=\frac{7}{2}\\ & \text{Story books = 1512}\\ & \text{7 units}\to 1\text{512}\\ & \text{1 unit}\to \frac{1512}{7}=216\\ & \text{2 units}\to 216\times 2=432\\ & \text{Other books }=432\\ & \text{New ratio of}\\ & \frac{\text{Story books}}{\text{Other books}}=\frac{15}{4}\end{array}$$
As we know that only story books are added
$$\begin{array}{rl}& \text{4 units}\to 43\text{2}\\ & \text{1 unit}\to \frac{432}{4}=108\\ & \text{15 units}\to 108\times 15\\ & =1620\end{array}$$
New collection of story books = 1620
∴ Number of story books are added = 1620 - 1512 = 108