A gaseous hydrocarbon (V.D. =15) having volume 10 ml was exploded with excess of $O_2$ .On cooling, a contraction in volume by 25 ml was observed. If the molecular formula of the hydrocarbon is $C_x{H}_y$, then find the value of x+y

Let the molecular formula of bydrocarbon $=C_x{H}_y$
Reaction  $\underset{\begin{array}{l} \\ \\ 10ml\end{array}}{C_x{H}_y}+\underset{\begin{array}{l} \\ 10(x+\frac{y}{4})ml\end{array}}{(x+\frac{y}{4})O_2}\to \underset{\begin{array}{l} \\ \\ 10xml\end{array}}{xC{O}_2}+\frac{y}{2}{H}_{2}O$
From reaction contraction in volume is given as: 
 $10+10(x+\frac{y}{4})-10x=25\left(given\right)$   $\therefore y=6$
Now, Formula of hydrocarbon $=C_x{H}_6$
Mol. wt. of  $C_x{H}_6=(12\times x)+(6\times 1)=2\times 15$
 $\therefore x=2$
Hence, Molecular formula of hydrocarbon $=C_2{H}_6$