求教两道陈向东数学中的题目

1. The arithmetic mean of a data set is 46 and the standard deviation of the set is 4. Which value is exactly 1.5 standard deviations from the arithmetic mean of the set?

A.42 B.44.5 C.47.5 D.50 E.52

我只做出来该数要大于46+2.5根号n，也就是C或者D的选项，但是答案是E

2.If n is positive integer and n平方 is divisible by 72, then the largest positive integer that must divide n is

A.6  B.12  C.24  D.36  E.48

我选了A，答案是B

3.A merchant sells an item at a 20 percent discount, but still makes a gross profit of 20 percent of the cost. What percent of the cost would the gross profit on the item have been if it had been sold without the discount?

A.20%  B.40%  C.50%  D.60%  E.75%

答案是C，请教如何求解？

谢谢！！！