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, , Marble4, , , Marble7, Marble8, , Marble 10.
Credit to a book I got. Intro to Enumerative Combinatorics by Miklos Bona.
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There are ( 4 1 0 ) = 2 1 0 ways to choose the spots for the red, blue, green, and orange marble.
For the remaining 6 spots, there are 6 ! = 7 2 0 ways to arrange these marbles.
Since these are independent of each other, we can simply multiply to obtain a result of 2 1 0 ⋅ 7 2 0 = 1 5 1 2 0 0 .