The price (per litre) of petrol increases by 85%. By what percent should its consumption be reduced such that the expenditure on it increases by 11% only?

Correct option is C

Given:

The price of petrol increases by 85%.
The expenditure on petrol increases by 11%.
Formula Used:
Expenditure = Price × Consumption
Solution:
Let the original price of petrol be  P and the original consumption be  C.
Original expenditure =$$P \times C .$$​
After the price increase:
New price = P + 0.85P = 1.85P 
Let the new consumption be C' .
New expenditure =$$1.85P \times C'.$$​
New expenditure = Original expenditure + 48% of original expenditure
​$$1.85P \times C' = 1.11\times (P \times C) .$$​​
C' =$$0.60 \times C$$​ 
Reduction = C - C' = C - 0.60C = 0.40C .
Percentage reduction =$$\frac{0.40C}{C} \times 100\% = 40\%$$

Alternate Method:

The price of petrol increases by 85%.

Then, 100 + 85 = 185

The expenditure on petrol increases by 11%.

100 +11 = 111

So, Reduction= 185 - 111 =74

Percentage reduction =$$\frac{74}{185}\times 100 = 40\%$$