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Posted by on Monday, December 5, 2011 at 7:24pm.

Rodney and James are each 30 feet from a door. Rodney moved one foot closer and James moved half the distant between himself and the door. If each person continued to move in this fashion, who would be closer to the door after 10 moves? Who reaches the door first?

  • math - , Monday, December 5, 2011 at 10:05pm

    In 10 moves, Rodney has covered 10 feet , so he is 20 ft from the door
    in 10 moves , James has moved:
    15 + 15/2 + 15/4 + 15/8 + 15/16 + .. + 15/2^9
    this is a geometric series with a= 15, r = 1/2
    sum(10) = 15(1 - (1/2)^10)/(1 - 1/2)
    = (1023/1024)(30)
    so he is (1/1024)(30) feet away from the door
    notice on the 10th move, he is 1/2^10 of the distance away.

    moving in this fashion , James will never get to the door
    e.g., after 1000 moves he would still be (1/2^1000) of the distance away from the door
    whereas Rodney will clearly reach the door after 30 moves.

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