The present age of son is half of the present age of his mother. Ten years ago, his mother's age was thrice the age of her son. What is the present age of the son ?

Answer: Option A

Let mother's age be 2x years,
Then, son's age = x years
$$\begin{array}{rl}& \therefore \left(2x-10\right)=3\left(x-10\right)\\ & \Rightarrow 2x-10=3x-30\\ & \Rightarrow x=20\end{array}$$
Son's age = 20 years