So I was given an assignment with 50 questions.

And I dont know how to do this one:

One evening, Andy, Barb and Carol play three rounds of a betting game with the understanding that the person who comes last in each round must double the money of the other two. After thrree rounds, each person has been last just once and each has $24. The amount that Andy started with at the beginning of the evening could have been:

(A) 36 (B) 37 (C) 38 (D) 39 (E) 40

And could you explain why?

Thanks ^_^