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Math Challenge II-A Combinatorics Question 8.29

 
 
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Re: 回复: Re: Math Challenge II-A Combinatorics Question 8.29
by Dr. Kevin Wang - Thursday, 7 October 2021, 1:37 AM
 

To pick 4 balls without replacement, and with at least one of each color, there are 3 cases: (1) 2 red, 1 green, and 1 yellow; (2) 1 red, 2 green, and 1 yellow; (3) 1 red, 1 green, and 2 yellow.  These 3 cases do not overlap, so you can count them separately, and add them up.