Let the price of 1 litre of petrol be Rs. $x$ and let Raj initially buys '$y$' litres of petrol.
Therefore, he would have spent Rs. $xy$ on petrol.
When the price of petrol increases by $25\%$, the new price per litre of petrol is $1.25x$.
Raj intends to increase the amount he spends on petrol by $15\%$.
i.e., he is willing to spend $xy + 15\%$ of $xy = 1.15xy$
Let the new quantity of petrol that he can get be '$q$'.
Then, $1.25x \times q = 1.15xy$
As the new quantity that he can buy is $0.92y$, he gets $0.08y$ lesser than what he used to get earlier.
Or a reduction of $8\%$.
Edit: For an alternative solution, check comment by Radheshyam Shukla.
Edit 2: For yet another alternative solution, check comment by Chirag Goyal.
Edit 3: For yet another shortcut alternative solution, check comment by Karan.