An urn contains 9 balls two of which are red, three blue and four black. Three balls are drawn at random. Find the probability that they are of the same color.
Balls in urn = 9
3 balls can be drawn in 9C3 ways.
To draw the balls of same color, we can select balls all three red, all blue or all black.
P(ball of same color) = P(all blue) or P(all black) or P(all red)
P(all red) = not possible as we have to select 3 balls but there are only 2 red balls.
∴ P(All red) = 0
P(all black) 
And P(all blue)
∴ P(ball of same colour) = P(all blue) or P(all black) or P(all red)
⇒ P(ball of same colour) = P(all blue) + P(all black) + P(all red)
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