请教GWD数学题

yifeir

1、Before a certain tire is used, 40 percent of its total weight consists of tread.  If during a lifetime of use, 50 percent, by weight, of the tire’s tread is lost and no other parts of the tire is lost, what per cent of the tire’s total remaining weight consists of the remaining tread?

 

A.   55%

B.   20%

C.   25%

D.      30%

E.       35%

为什么答案会选C？他一开始包括40%的tread，那后来即便是用了50%，不还是应该在剩下的里面有40%的么？

2、If x and y are nonzero integers, is 18 a factor of xy²?

(1)     x is a multiple of 54.

(2)     y is a multiple of 6.

答案选D。不懂为什么2也可以单独判断呢？只是6的倍数不一定能被18整除啊？

3、A certain circle in the xy-plane has its center at the origin.  If P is a point on the circle, what is the sum of the squares of the coordinates of P?

(1)     The radius of the circle is 4.

The sum of the coordinates of P is 0.

答案选A，没有懂这道题目意思。

4、One kilogram of a certain coffee blend consists of x kilogram of type I coffee and y kilogram of type II coffee.  The cost of the blend is C dollars per kilogram, where C = 6.5x + 8.5y.  Is x < 0.8?

(1)     y > 0.15

(2)     C ≥ 7.30

答案选B。为什么只需要C的值就可以了呢？我觉得选C啊。

问的题目有点多，谢谢大家。数学比较差。大家见谅。多谢多谢。

clearwater

1. 0.2/(1-0.2)=0.25

2.因为是xy^2, y^2=36是18的倍数亚~

3.就是P点的坐标的平方和是多少,因为P点在题目所说的圆上,满足x^2+y^2=r^2

所以 只要知道r 就可以知道x^2+y^2

4.因为x+y=1

所以可以把y=1-x代入 6.5x+8.5y≥7.3 就可以解得

yifeir

谢谢你耐心的回答。呵呵。