Define the relationship

Let initial price be $P$ and initial consumption be $C$. Initial expenditure $E = P \times C$. New price $P' = P - 0.2P = 0.8P$.

$P' = 0.8P$

Set constant expenditure

To keep expenditure $E$ unchanged, let new consumption be $C'$. So, $0.8P \times C' = P \times C$.

$C' = \frac{C}{0.8} = 1.25C$

Calculate percentage increase

The increase in consumption is $C' - C = 1.25C - C = 0.25C$. Converting this to a percentage relative to $C$: $(0.25/1) \times 100 = 25\%$.

$\text{Increase} = 25\%$