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In a study of Earth's energy balance, X% of incoming solar radiation reaches the Earth's surface, and Y% of that incoming solar radiation is absorbed by greenhouse gases in the atmosphere. What is the value of (X - Y)?

1. X - Y
2. Y - X
3. X + Y
4. 100 - (X + Y)

Correct answer: X - Y

Solution

Since X% of sunlight reaches the surface and Y% is absorbed by greenhouse gases, the difference (X - Y) is exactly the expression given in option A. The question tests whether students recognise that the answer is the algebraic identity itself.

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