Marbles in a Certain Order

Suppose we have 10 marbles, each of different color. If we are to arrange them, from left to right, in how many ways can we have the red one, blue, green, and orange, arranged from left to right?

Example: Marble1, Marble2, R \textcolor{#D61F06}{R} , Marble4, B \textcolor{#3D99F6}{B} , G \textcolor{#20A900}{G} , Marble7, Marble8, O \textcolor{#EC7300}{O} , Marble 10.

Credit to a book I got. Intro to Enumerative Combinatorics by Miklos Bona.

151204 151202 151206 151200

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2 solutions

Henry U
Mar 8, 2019

There are ( 10 4 ) = 210 \binom {10}{4} = 210 ways to choose the spots for the red, blue, green, and orange marble.

For the remaining 6 6 spots, there are 6 ! = 720 6! = 720 ways to arrange these marbles.

Since these are independent of each other, we can simply multiply to obtain a result of 210 720 = 151200 210 \cdot 720 = \boxed{151200} .

Jay B
Mar 8, 2019

There are 10! rearrangements without restrictions. Each of those would contain the desired sequence with the other marbles arranged somehow. Keep those fix and start permuting the red, blue, green and orange ones. These permutations would have all but 1 impossible cases. Then there is a 4!-to-1 function from the set of all 10! permutations into the set of all permutations where we must have R , B , G , O \textcolor{#D61F06}{R}, \textcolor{#3D99F6}{B}, \textcolor{#20A900}{G}, \textcolor{#EC7300}{O} in that order. Using the division principle, we have 10 ! / 4 ! = 151200 10!/4!=151200 desired arrangements.

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