求一道数学DS题，不知道出处，是在XDF老师讲义里的

歆欣12388

at the beginning of a five-day trading week, the price of a certain stock was $10. during the week, four of the five closing prices of the stock exceeded $10.Did the average closing price of the stock during the week exceed its price at the beginning of the week?
(1)The stock's closing price on Tuesday was the same as its closing price on Thursday.
(2)The sum of the stock's highest and lowest closing prices during the week was 20.
看到stock这类题目就有点慌，不懂他讲什么，怎么做？？？？

歆欣12388

有没有人帮忙解答下，答案是B

sdcar2010

P  Mon  Tue  Wed  Th  Fri
b  10                                 at least 4 over 10
c                                        4 over 10, 1 below

Will the average of c over 5 day greater than 10?

1) c(Tue) = c(Th).  Not sufficient since if the only closing price below 10 is 0, and all the closing price above 10 is 11, the average is still below 10.

2) The sum of the stock's highest and lowest closing prices was 20.  Bingo.  The condition ensures that the sum of the five day closing price is above 50 (20 plus three over 10 prices).  Therefore, the average of closing prices for the week is definitely over 10.

歆欣12388

P  Mon  Tue  Wed  Th  Fri
b  10                                 at least 4 over 10
c                                        4 over 10, 1 below

Will the average of c over 5 day greater than 10?

1) c(Tue) = c(Th).  Not sufficient since if the only closing price below 10 is 0, and all the closing price above 10 is 11, the average is still below 10.

2) The sum of the stock's highest and lowest closing prices was 20.  Bingo.  The condition ensures that the sum of the five day closing price is above 50 (20 plus three over 10 prices).  Therefore, the average of closing prices for the week is definitely over 10.

-- by 会员 sdcar2010 (2010/11/9 11:01:59)

thanks a lot, I get it ...at first, I think of "exceeded 10" means increased by 10,that is 10+10, now I know it means "greater than 10"...