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If 21 k C C C 20 1 i 3 1 i 20 i 20 1 i 20 = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + ∑ = − − then k is equal to

1. 100
2. 200
3. 50
4. 400

Correct answer: 100

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