Question 1202292
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A box contains 5 red, 4 blue, and 3 white balls. 
In how many ways can we select 4 balls such that exactly 2 are blue?
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So, there are 4 blue balls and 5+3 = 8 not-blue balls in the box.


We can select 2 blue balls from 4 blue balls in  {{{C[4]^2}}} = {{{(4*3)/2)}}} = 6 different ways.

We can select 2 not-blue balls from 8 not-blue balls in  {{{C[8]^2}}} = {{{(8*7)/2)}}} = 28 different ways.


Thus we can comprise a quadruple consisting of 2 blue and 2 not-blue balls in

    {{{C[4]^2*C[8]^2}}} = 6*28 = 168  different ways.    <U>ANSWER</U>
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Solved.