Mike523X 发表于 2018-8-1 13:55
求助 这个题 怎么解啊
There are 2 black balls, one red ball and one green ball, identical in shape a ...
4!/2!=12种
个人的解法:
只考虑G and R, put into order, there will be 4!/2!=12 ways
since the 2 black balls are identical the same, there is no necessary to distinguish bewteen them.
The answer should be 12