In a box there are 8 red, 7 blue, and 6 green balls. One ball is picked up randomly. What is the probability…

It can be noted that if the ball is neither red or green, then it must be blue.

Number of blue balls $=7$

Now, total number of balls $=8+7+6=21$

Thus, the probability is $=\frac{7}{21}=\frac{1}{3}$