The price (per litre) of petrol increases by 80%. By what percent should its consumption be reduced such that the expenditure on it increases by 17% only?

Correct option is C

Given:

Percentage increase in price = 80%

Percentage increase in expenditure = 17%

Formula Used:

If the price of a commodity increases by x%, and the expenditure is allowed to increase by y%,

​$$\text{Percentage decrease} = \left( \frac{x - y}{x + 100} \right) \times 100$$​

Solution:

​$$\text{Percentage decrease} = \left( \frac{80 - 17}{80 + 100} \right) \times 100$$​

​$$\text{Percentage decrease} = \left( \frac{63}{180} \right) \times 100 = 35\%$$ 
Alternate Solution: 

Let:

Original price = 100

Original consumption = 100

=>Original expenditure = 100 × 100 = 10,000

New price = 100 + 80 = 180

New expenditure = 10,000 × 1.17 = 11,700

New consumption = $$\frac{11700}{180}$$ = 65

Reduction = 100 - 65 = 35

Required reduction % = $$\frac{100 - 65}{100} \times 100 = 35\%$$

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