can i get the answers of R.S Aggarwal of class 9th?

Let x = cost price and g= actual gain percentage.
Then profit = g% of x =gx/100
So, selling price = cost price + profit=78
$$\begin{gathered} x+\frac{gx}{100}=78 \\ \frac{100x+g}{100}=78 ...\left(1\right) \end{gathered}$$
New gain = 20% more than g = 20+g
selling price = cost price + profit=83
$$\begin{gathered} x+\frac{(20+g)x}{100}=83 \\ \frac{100x+gx+20x}{100}=83 \\ \frac{100x+gx}{100}+\frac{20x}{100}=83 \\ 78+\frac{20x}{100}=83 (from (1\left)\right) \\ \frac{20x}{100}=83-78=5 \\ x=\frac{5*100}{20}=\frac{500}{20}=Rs. 25 \end{gathered}$$