[求助]两数学题

bioliwu

1，一月机经４９

For some telephone call record, the average daily phone call range in July is 7, the average daily phone call range in Agust is 10, what is the minimum range of daily phone call for July and August? answer: 10

不知道这个怎么得来的，看了讨论也没明白

２,from gmatprep

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 result of the survey had a mean of 21 hours and a standard deviation of 6 hours. The number of hours of that participated, a resident of town x telephone last week was between 1 and 2 standard deviations below the mean. Which of the following could be the number of hours the participated watched television last week?

a.30 b.20 c.18 d.12 e.6 answer:d

上面的斜体怎么理解，不知道怎么算

kenlily

1. the key to this problem is minimum RANGE. July: 1-7 calls. Aug: 1-10 calls.

So the range of numbers of calls for two month is 1-10, 10, which is the minumum.

Then, lets suppose July: 1-7, but Aug is 5-15 (range still 10, and meaning 5-10 calls daily). But now the range of calls in July and Aug is 15, which is not the minumum the question asks.

apple1

２,from gmatprep
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 result of the survey had a mean of 21 hours and a standard deviation of 6 hours. The number of hours of that participated, a resident of town x telephone last week was between 1 and 2 standard deviations below the mean. Which of the following could be the number of hours the participated watched television last week?
a.30 b.20 c.18 d.12 e.6 answer:d

斜体的大意是：参与调查的一位X镇居民上周看电视的时间在低于平均数的1个到2个标准差之间。

所以该人上周看电视的时间在21-2*6＝9到21-6＝15之间，因此选D