发觉数学好难，prep里三道题求点拨

mulluer

If n is a positive integer and product of all the integers from 1 to n,inclusive,is a multiple of 990,what is the least possible value of n?

A10 B11 C12 D13 E14

答案是B

During a 40-mile trip,Maria traveled at a average speed of x miles per hour for the fisrt y mile of the trip and at an average speed of 1.25x miles per hour for the last 40-y miles of the trip. The time that Maria took to travel the 40 miles was what percent of the time it would have taken her if she had travled at anaverage speed pf x miles per hour for the entire trip?

(1) x=48

(2)y=20

答案是2

Stock   Number of the share

V            68

W          112

X             56

Y             94

Z             45

The table shows the number of shares of each of the 5 stocks owned by Mr.Sami. If Mr.Sami was to sell 20 shares of Stock X and buy 24 shares of stock Y,what would be the increase in the range of the numbers of shares of the 5 stocks owned by Mr.Sami?

A4 B6 C9 D 15 E20

答案D

mulluer

怎么会prep里的题都狂错,受打击了

第一道做不出来，

第二道我选的是3，两个独立都可以

第三道题更加郁闷的一头雾水，我选的是A。。

benzine

1. a multiple of 990意味着product of all the integers from 1 to n,inclusive至少是990，即990的1倍。而990= 9*10*11，11是质数，且9=3*3, 10= 2*5，所以n最小为11就可以满足题干中的条件。

2. 根据题意，percent = ((y/x+(40-y)/1.25x)/(40/x)，经化简可得 percent = (y+(40-y)/1.25)/40，所以percent只与y值有关。

3. 设在Sami进行买卖操作之前的range为M，买卖操作之后的range为N，则可得M = 102 - 45 = 67，N = 118 - 36 = 82，而N - M = 82 - 67 = 15

建议LZ再看看仔细数学的概念