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Bunuel
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sdas80923
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GMAT Focus 1: 655 Q83 V85 DI80
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Why is this approach incorrect?

Choosing AP with difference 1 [x]: (22/24)*(1/23)*(1/22)
Choosing AP with difference 2 [y]: (20/24)*(1/23)*(1/22)

Final answer= x+y
The approach is incorrect because it assumes a specific order of drawing (e.g., drawing 1, then 2, then 3), while the question is about combinations, not sequences. Each valid AP set like {1, 2, 3} can appear in 6 different orders, so the method undercounts the total. The correct approach counts unordered sets. If you multiply the result you get by 3! (i.e., 6), you'll arrive at the correct answer.
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