请教gwd 数学题

8-2 A certain characteristic in a large population has a distribution that is symmetric about the mean m.If 68%of the distribution lies within one standard deviation d of the mean, what percent of the distribution is less than m+d ?

key : 84%

6-22 When tossed ,a certain coin has equal probability of landing on either side.If the coin is tossed 3 times ,what is the probability that it will land on the same side each time ?

key : 1/4

6-3 Each participant in a certain study was assigned a sequence of 3 different letters from the set {A,B,C,D,E,F,G,H}.If no sequence was assigned to more than one participant and if 36 of the possible sequences were not assigned ,what was the number os participants in the study ? (Note,for example,that the sequence A,B,C is different from the sequence C,B,A)

key :300

概率的题每次都搞不懂 帮帮忙啦 非常感谢