x    x

  x  x   x

 x  x  x  x

x  x  x  x  x

 凹 凹 凹 凹

上图的第一行的“o”表示一个小球，第2、3、4行得x表示障碍物，第六行的凹表示小凹槽。小球从上掉下掉到第3行时受中间的障碍物阻滞，向左或向右的几率个百分之五十。以下亦然。问：最后掉到第6行的第2个小凹槽的几率有多大？

解题思路是什么，实在看不懂啊。十分感谢。

3rd row:
left: 50%
right: 50%

4th row:
left = 3rd left * 50% = 50%*50% = 25%
middle = 3rd left * 50% + 3rd right * 50% = 50%*50% + 50%*50% = 50%
right: 25% (same as left)

5th row:
left = 4th left * 50% = 25% * 50%
middle left: 4th left * 50% + 4th middle * 50%
middle right = 4th middle * 50% + 4th right * 50%
right = 4th right * 50%

See if you can work out the 6th row, =)