In a new budget, the price of petrol rose by 25%. By how much percent must a person reduce his…

Expenditure is price times quantity. If the price becomes 1.25 times as much, the quantity must become $1/1.25=0.8$ times as much for the expenditure to stay the same, which is a 20% reduction. A 25% cut is wrong because $1.25\times0.75=0.9375$, which would lower the expenditure rather than hold it steady.