1, the fourth person bought two tickets, which was half of that before the third person bought them, so the third person had them before. 2X(2+ 1/2)=5。

2, the second person bought half of the first one, so the rest is half of the first one (that is, the number before the third one). So before the second person buys, it is 2X(5- 1/2)=9.

Similarly, the first person bought less than half of the total, so the rest is more than half of what the first person bought (that is, the number before the second person bought). So the total is 2X(9- 1/2)= 17.