【求助】排列组合概率文盲求解题

徐小玉儿

GWD数学
A gardener is going to plant 2 red rosebushes and 2 white rosebushes.  If the gardener is to select each of the bushes at random, one at a time, and plant them in a row, what is the probability that the 2 rosebushes in the middle of the row will be the red rosebushes? 答案1/6

A certain roller coaster has 3 cars, and a passenger is equally likely to ride in any 1 of the 3 cars each time that passenger rides the roller coaster.  If a certain passenger is to ride the roller coaster 3 times, what is the probability that the passenger will ride in each of the 3 cars?  2/9
好像很简单 但是还是无从下手 错的节奏
下周考试……目测要换题 没啥JJ
再问下这类的解题技巧……GWD版的有无详细解释的文档？