Explanation:

To determine the percentage decrease in consumption needed to keep expenditure the same:

- Original expenditure on apples = Rs 80 per kg.

- New price = Rs 100 per kg.

- Let the original quantity be Q and expenditure be E = 80Q.

- With the new price, to maintain the same expenditure: 100 x new quantity = 80Q.

- New quantity = 0.8Q.

- Percentage decrease = [(Q - 0.8Q)/Q] x 100 = 20%.

Option 3: 20 is the correct answer.

- Option 1: 25 is incorrect; it overestimates the decrease.

- Option 2: 22 is incorrect; it doesn't match calculations.

- Option 4: 18 is incorrect; it underestimates the decrease.

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