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How many collections of 12 Skittles have at least one of each color? (There are 5 colors of Skittles.)

 Feb 27, 2021
 #1
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Let the number of Skittles of each color be a, b, c, d, and e. We have 12 skittles in total:

 

a + b + c + d + e = 12

 

Note that we also need at least one of each color, so we can substitute a' = a + 1, b' = b + 1, and so on, to ensure that each color has at least one Skittle.

 

a' + b' + c' + d' + e' = 7

 

This is an example of stars and bars, and the ways to choose is (7+5-1) choose (7-1) = 462.

 Feb 28, 2021

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