math bible 的排列组合题！！

LuLuanna · 发表于 2017-3-15 21:22:29

1.The University of Maryland, University of Vermont, and Emory University have each 4 soccer
players. If a team of 9 is to be formed with an equal number of players from each university, how
many number of ways can the selections be done?
(A) 3
(B) 4
(C) 12
(D) 16
(E) 25
Selecting 3 of 4 players can be done in 4C3 =4! /3!?1! = 4 ways.
The selection from the 3 universities can be done in 3 x? 4 = 12 ways.
The answer is (C).

为什么不是4x4x4=64 而是4+4+4=12 呢？
--------------------------------------------------------------------------------------------------------------------------------------------

2. In how many of ways can 5 balls be placed in 4 tins if any number of balls can be placed in any tin?
(A) 5C4
(B) 5P4
(C) 5^4
(D) 4^5
(E) 5^5
The first ball can be placed in any one of the four tins.
Similarly, the second, the third, the fourth, and the fifth balls can be placed in any one of the 4 tins.
Hence, the number of ways of placing the balls is 4 x? 4 ?x 4 x? 4 ?x 4 = 4^5. The answer is (D).
Note: We used Model 2 here.

为什么不能是第一个tin有0,1,2,3,4,5这五种装球方法，以此类推，即the number of ways of placing the balls is 5^4呢？

爱偷拍的豆腐魚 · 发表于 2017-3-16 00:36:46

关于第二题的解释：之所以是4^5，是因为一个tin里可以放0-5个ball，但是一个ball只能放入一个tin。
第一题：我觉得也是64哈哈哈哈哈哈

math bible 的排列组合题！！

所属分类: GMAT考试

正在浏览此版块的会员 ()