Permutation or Combination
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Permutation or Combination

[From: ] [author: ] [Date: 11-08-07] [Hit: ]
r2,r3,...n!......
Find the number of different ways of placing 15 balls in a row given that 4 are red, 3 are yellow, 6 are black and 2 are blue.

My own calculation is 6912 but I don't have the real answer.

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the permutation formula when r1,r2,r3,... objects from n are identical is

n!/(r1*r2!*r3!....) , so

# of ways = 15!/(4!*3!*6!*2!) = 6,306,300 <--------

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just as you have to multiply to account for different objects,
you need to divide to "take out" arrangements that are the same

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15!/(4!3!6!2!)=900900 by my reckoning
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