Pls answer this Q....

Dear Student, 
Solution) Gaurav's age = x years and Saurav's age = y years
Therefore,  ATQ,
$$\begin{gathered} \frac{x}{y}=\frac{5}{7} \\ 7x=5y \\ 7x-5y=0 \end{gathered}$$
Now, after 3 years, 
$\frac{x+3}{y+3}=\frac{4}{5}$
5x + 15 = 4y + 12
5x - 4y = 12 - 15
5x - 4y = - 3
So, the option B is correct. 
Regards!