The price of cooking oil increases by 25%. By what percentage should a family reduce its use of cooking oil so that the expenditure on it does not increase?

The use should be cut by 20%.

To keep expenditure fixed, reduction % = increase / (100 + increase) x 100.

Here increase = 25%.

Reduction = 25 / 125 x 100.

25 / 125 x 100 = 20%.

So the family should reduce use by 20%.