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 48% only?

Let the original price per litre be $x$ and original consumption be $1$ litre.
After an $85%$ increase: \quad $x \times (1 + 0.85) = 1.85x$
Let consumption be reduced by $y%$: \quad new consumption $= 1 - \dfrac{y}{100}$
New expenditure: \quad $1.85x \times \left(1 - \dfrac{y}{100}\right)$
According to the question this equals $1.48$ times the original expenditure:
$1.85x \times \left(1 - \dfrac{y}{100}\right) = 1.48x$
$1.85\left(1 - \dfrac{y}{100}\right) = 1.48$
$1 - \dfrac{y}{100} = \dfrac{1.48}{1.85} = 0.8$
$\dfrac{y}{100} = 1 - 0.8 = 0.2$
$y = 20$