If 3a + 5b3a - 5b = 5, then a : b is equal to?

Answer: Option D

$$\begin{array}{rl}& \frac{3a+5b}{3a-5b}=5\\ & \Rightarrow 3a+5b=15a-25b\\ & \Rightarrow 12a=30b\\ & \Rightarrow 2a=5b\\ & a:b\\ & 5:2\end{array}$$