To determine how much a person should reduce their petrol consumption in response to an 80% increase in price while keeping their expenditure constant, we can follow a systematic approach.

Understanding the Price Increase
- If the original price of petrol is denoted as \( P \), an 80% increase means the new price becomes:
\[
\text{New Price} = P + 0.8P = 1.8P
\]

Expenditure and Consumption Relationship
- Let’s assume the person originally consumes \( C \) liters of petrol. Therefore, the initial expenditure is:
\[
\text{Expenditure} = P \times C
\]
- To maintain the same expenditure after the price increase, we denote the new consumption as \( C' \). The equation for expenditure becomes:
\[
\text{Expenditure} = 1.8P \times C'
\]

Setting the Expenditures Equal
- To keep the expenditure the same, we equate the two expressions:
\[
P \times C = 1.8P \times C'
\]
- Simplifying this by canceling \( P \) from both sides (assuming \( P \neq 0 \)) gives us:
\[
C = 1.8C'
\]

Calculating New Consumption
- Rearranging the equation to find \( C' \):
\[
C' = \frac{C}{1.8} \approx 0.5556C
\]

Determining the Reduction Percentage
- The reduction in consumption can be calculated as:
\[
\text{Reduction} = C - C' = C - 0.5556C = 0.4444C
\]
- To find the percentage reduction:
\[
\text{Percentage Reduction} = \left( \frac{0.4444C}{C} \right) \times 100 \approx 44.44\%
\]

Conclusion
- Therefore, to maintain the same expenditure after an 80% increase in petrol prices, the person should reduce their consumption by approximately **44.44%**.