Oh! Its not the age to solve this , a small child can do this

S = age of Sonal at present

R = age of Raj at present

$\dfrac{S}{R}=\dfrac{2}{3}$ $\implies$ $3S=2R$ $(1)$

$S+R=20$ $\implies$ $S=20-R$ $(2)$

Substitute $(2)$ in $(1)$ ,

$3(20-R)=2R$ $\implies$ $60-3R=2R$ $\implies$ $60=5R$ $\implies$ $R=12$

It follows that $S=20-12=8$

The difference of their ages 12 years from now is equal to the difference of their ages at present. So the desired answer is $12-8=\boxed{4}$

let ages be 2x and 3x..... Now 2x + 3x = 20, 5x=20 ,x=4 Now, 2 X 4 = 8 and 3 X 4 = 12...after 12 years 8 + 12 = 20 and 12 +.12 = 24.Now, differnece 24- 20 = 4 (answer)