Zeno's tank holds 24 gallons of water. Zeno starts filling the tank, but when it is halfway full he decides to start draining water out. Once half of the water that he added is drained, he decides to add back half of the water that he just drained. He then drains half of the water that he just added, and continues alternately adding or draining half of the previous quantity of water. After 100 cycles of adding and draining water, how many gallons of water are in the tank? Express your answer to the nearest whole number.
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The volume in gallons after n th cycle of adding and draining is given by:
V n = Adding ( 1 2 + 3 + 4 3 + ⋯ + 4 n − 1 1 2 ) − Draining ( 6 + 4 6 + 4 2 6 + ⋯ + 4 n − 1 6 ) = Remaining after n th cycle ( 6 + 4 6 + 4 2 6 + ⋯ + 4 n − 1 6 ) = 6 k = 0 ∑ n − 1 ( 4 1 ) k = 6 × 1 − 4 1 1 − 4 n 1 = 8 ( 1 − 4 n 1 )
⟹ V 1 0 0 = 8 ( 1 − 4 1 0 0 1 ) ≈ 8