求教两道陈向东数学中的题目

heice

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！！！

ilhht

第一个题目，求出的应该是46-1.5*4 and 46+1.5*4，所以答案E正好是52

第2个题目，n的平方可以被72整除，就是说明N＊N中必然包含了72的所有因子。把72分解质因数72＝2＊2＊2＊3＊3由于N是一个正整数。由此可见，72＝2＊（2的平方）＊（3的平方），还有一个独立的2不能配成一个整数的平方，所以N的平方至少是72的2倍，所以得出，n＝2＊2＊3就是12了。希望可以帮到楼主

[此贴子已经被作者于2009-3-8 22:37:36编辑过]

ilhht

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%

这个题目，首先卖的时候20％off proft＝sale price－cost 所以假设原来的卖价是X，那么cost *20％＝X（1－20％）－cost 所以得出 1.2cost=0.8X,  X=3/2cost

proft=x-cost=3/2cost-cost=1/2cost 所以就是50％，选C希望可以帮到楼主