Explanation:

Let initial rate of Petrol is 100 Rs./litre and we buy 10 litres (Suppose)

So the total expenditure = 100 * 10 = 1000 Rs.

Final  Price of Petrol after a hike of 25%  is 125 Rs./litre.

Quantity now bought = 'x'  (Suppose)

New  Expenditure = 125*x = 1000 Rs. (Since it  is constant)

x = 1000/125  =  8 

We used to buy 10 litres earlier. Now we buy only 8.

So %age decrease =  (10 - 8)/10 * 100% = 20%       (He  should reduce the consumption by 20%)