The age of a father 10 years ago was thrice the age of his son. 10 years hence , the father's age will be twice that of his son. The ratio of their present age is = ?

Answer: Option B

Let son's age 10 years ago be $$x$$ years
Then, father age 10 years ago = 3$$x$$ years
Son's age now = ($$x$$ + 10) years,
Father age now = (3$$x$$ + 10) years
$$\begin{array}{rl}& \left(3x+10\right)+10=2\left[\left(x+10\right)+10\right]\\ & \Rightarrow 3x+20=2\left(x+20\right)\\ & \Rightarrow 3x+20=2x+40\\ & \Rightarrow x=20\end{array}$$
Ratio of present ages of father and son
$$\begin{array}{rl}& \text{ = }\frac{3x+10}{x+10}\\ & =\frac{3\times 20+10}{20+10}\\ & =\frac{70}{30}\\ & =\frac{7}{3}\\ & =7:3\end{array}$$