Answer is 20 i.e. we can see it using an example

Price * Consumption Expenditure i.e. If price and consumption are 10 each then expenditure will be 100 and if price increases by 25 means it become 12.5 so in order to make expenditure 100 we have to divide 100 by 12.5 i.e. 8 so consumption becomes 8 hence it is reduced by 20 i.e (12.5 * 8 100)