PREP DS2-7题, 求助

springsound

7. 462-!-item-!-187;#058&000279

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B是怎么得来的呢? 谢谢.

ashleywang86

由x+y=1得到x=1-y

代入C = 6.5x + 8.5y

由C >= 7.30，解出X<=0.6,充分。。。

但是条件一，只能得出X<0.85,不一定小于0.8，不充分，所以选B

springsound

谢谢ashleywang86:

终于明白了: 原来有一个x+y=1的隐含条件....