试题与答案

设有编号为1,2,3,4,5的五个茶杯和编号为1,2,3,4,5的五个杯盖,将五

题型:选择题

题目:

设有编号为1,2,3,4,5的五个茶杯和编号为1,2,3,4,5的五个杯盖,将五个杯盖盖在五个茶杯上,至少有两个杯盖和茶杯的编号相同的盖法有(  )

A.30种

B.31种

C.32种

D.36种

答案:

假设相同的盖和杯5个全部都是,那么只有一种,假设相同的盖和杯4个,那么有0种,

假设相同的盖和杯3个,那么就有C

 35
×1=10种;

假设相同的盖和杯2个,那么就有C

 25
x2=20种;

所以:1+10+20=31种.

故选:B.

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