10号考，问几道PREP数学~希望大家帮忙~~

Blake1171

都是PREP的DS题。看解释看不懂的= =希望大家帮帮忙。。谢谢！

DS1
90. 8266-!-item-!-187;#058&005947
On Jane's credit card account, the average daily balance for a 30-day billing cycle is the average (arithmetic mean) of the daily balances at the end of each of the 30 days. At the beginning of a certain 30-day billing cycle, Jane's credit card account had a balance of $600. Jane made a payment of $300 on the account during the billing cycle. If no other amounts were added to or subtracted from the account during the billing cycle, what was the average daily balance on Jane's account for the billing cycle? (1) Jane's payment was credited on the 21st day of the billing cycle.
(2) The average daily balance through the 25th day of the billing cycle was $540.
这题完全读不懂啊TAT。。。

131. 13306-!-item-!-187;#058&009213
The positive integer k has exactly two positive prime factors, 3 and 7. If k has a total of 6 positive factors, including 1 and k,
18
what is the value of k ?
(1) 32 is a factor of k.
(2) 72 is not a factor of k.

165. 16466-!-item-!-187;#058&010795
Of the 1,400 college teachers surveyed, 42 percent said that they considered engaging in research an essential goal. How
many of the college teachers surveyed were women?
(1) In the survey, 36 percent of the men and 50 percent of the women said that they considered engaging in research an
essential goal.
(2) In the survey, 288 men said that they considered engaging in research an essential goal.
这题为啥是A。。。
我觉得应该是C啊。。。

169. 16735-!-item-!-187;#058&010875
If the integers a and n are greater than 1 and the product of the first 8 positive integers is a multiple of an, what is the value
of a ?
(1) an = 64
(2) n = 6
这题为啥是B？只知道N是6算不出来A吧应该。。。。。

214. 20789-!-item-!-187;#058&013258
The cost of a square slab is proportional to its thickness and also proportional to the square of its length. What is the cost
of a square slab that is 3 meters long and 0.1 meter thick?
(1) The cost of a square slab that is 2 meters long and 0.2 meter thick is $160 more than the cost of a square slab that is 2
meters long and 0.1 meter thick.
(2) The cost of a square slab that is 3 meters long and 0.1 meter thick is $200 more than the cost of a square slab that is 2
29
meters long and 0.1 meter thick.
同样。。解释都没看明白ORZ。。
大家帮帮忙TTTT谢谢啦。。。。。。

Blake1171

顶~~~