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Home  >>  CBSE XI  >>  Math  >>  Permutations and Combinations
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Find the number of ways of selecting 9 balls from 6 red balls,5 white balls and 5 blue balls if each selection consists of 3 balls of each colour.

$\begin{array}{1 1}(A)\;1000\\(B)\;1500\\(C)\;2000\\(D)\;2500\end{array} $

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1 Answer

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  • $C(n,r)=\large\frac{n!}{r!(n-r)!}$
The number of ways of selecting 3 red balls out of 6 red balls =$6C_3$
The number of ways of selecting 3 white balls out of 5 white balls =$5C_3$
The number of ways of selecting 3 blue balls out of 5 blue balls =$5C_3$
The number of ways of selecting 3 balls of each colour
$6C_3\times 5C_3\times 5C_3=6C_3\times 5C_2\times 5C_2$
$\Rightarrow \large\frac{6\times 5\times 4}{1\times 2\times 3}\times \frac{5.4}{1\times 2}\times \frac{5.4}{1.2}$
$\Rightarrow 20\times 10\times 10$
$\Rightarrow 2000$
Hence (C) is the correct answer.
answered May 14, 2014 by sreemathi.v
 

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