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All are different size ball so no two are of the same size and only one ball among each color of the ball is smallest so in the case of the smallest red ball
A total red ball
It is given that there are 6 red balls, 7 green balls and 5 blue balls. Therefore, the total number of balls = 18.
P(red ball) = 6/18.
P(smallest ball and red ball) = 1/18
Because out of all the 18 balls, only one is the smallest red.
Therefore, P(The smallest red ball is picked | a red ball is picked) = P(smallest ball and red ball)/P(red ball)
\frac{\frac{1}{18}}{\frac{6}{18}}
=1/6