（iii）Find the probability that the number of hours for which a randomly chosen student uses a games machine in a day is within 1.5 standard deviations of the mean．
（iii） 求随机选择的学生在一天内使用游戏机的小时数在平均值的 1.5 个标准差以内的概率。
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（b）The variable $Y$$Y$YY is normally distributed with mean $\mu$$\mu$mu\mu and standard deviation $\sigma$$\sigma$sigma\sigma ，where $4\sigma =3\mu$$4\sigma =3\mu$4sigma=3mu4 \sigma=3 \mu and $\mu \ne 0$$\mu \ne 0$mu!=0\mu \neq 0 ．Find the probability that a randomly chosen value of $Y$$Y$YY is positive．
（b） 变量 $Y$$Y$YY 呈正态分布，均值 $\mu$$\mu$mu\mu 为标准差 $\sigma$$\sigma$sigma\sigma ，其中 $4\sigma =3\mu$$4\sigma =3\mu$4sigma=3mu4 \sigma=3 \mu 和 $\mu \ne 0$$\mu \ne 0$mu!=0\mu \neq 0 。 求随机选择的值为 $Y$$Y$YY 的概率 为正。
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（a）Find the number of different arrangements of the 9 letters in the word DELIVERED in which the three Es are together and the two Ds are not next to each other．
（a）找到单词 DELIVERED 中 9 个字母的不同排列的数量，其中三个 E 在一起，两个 D 彼此不相邻。
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（b）Find the probability that a randomly chosen arrangement of the 9 letters in the word DELIVERED has exactly 4 letters between the two Ds．
（b） 找出单词 DELIVERED 中随机选择的 9 个字母排列在两个 D 之间正好有 4 个字母的概率。
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