PP1 - PS - Q 110 & 118 & 155

[sungtat]

小弟對這種類型的題目真的沒辦法
請大大幫忙一下
感謝


110. 11364-!-item-!-187;#058&007646 答案: D
The residents of Town X participated in a survey to determine the number of hours per week each resident spent watching television. The distribution of the results of the survey had a mean of 21 hours and a standard deviation of 6 hours. The number of hours that Pat, a resident of Town X, watched television last week was between 1 and 2 standard deviations below the mean. Which of the following could be the number of hours that Pat watched television last week?

(A) 30
(B) 20
(C) 18
(D) 12
(E) 6

118. 12240-!-item-!-187;#058&008164 答案: A
For a certain examination, a score of 58 was 2 standard deviations below the mean, and a score of 98 was 3 standard deviations above the mean. What was the mean score for the examination?

(A) 74
(B) 76
(C) 78
(D) 80
(E) 82


155. 16103-!-item-!-187;#058&010667 答案:D
If 2^x - 2^(x - 2) = 3(2^13), what is the value of x ?

(A) 9
(B) 11
(C) 13
(D) 15
(E) 17

我有找到155的解答
2^x - 2^(x - 2) = 3(2^13)
但是這段不太清楚...不知道怎麼 有 (2^2 - 1) ? 怎麼從 - 變成 *
=>2^(x-2)(2^2-1)=3(2^13)
=>x-2=13
=>x=15