The price of cooking oil has increased by 25%. By what percent should a family reduce the consumption of cooking oil so as not to increase the expenditure in this account?

20

Let,

Initial -------Price = 100     Quantity = 10         Expenditure (P x Q) = 1000

After --------Price = 125     Quantity = x [8]          Expenditure (P x Q) = 1000

So, 

Quantity = \(\frac{ 1000}{125 }\) = 8 

Initial qty = 10, New qty = 8

Reduction = \(\frac{2 }{10 }\) x 100 = 20%