What number has to be added to the terms 3 : 5 to make the ratio 5 : 6 ?

Answer: Option B

Let the number to be added be x
Then,
$$\begin{array}{rl}& =\frac{3+x}{5+x}=\frac{5}{6}\\ & \Rightarrow 6\left(3+x\right)=5\left(5+x\right)\\ & \Rightarrow 18+6x=25+5x\\ & \Rightarrow x=7\end{array}$$