Probability = no of favourable outcomes / no. of total possible outcomes. --(1)
Total number of balls = 16+20+24 = 60. Out of these, one ball is picked.
So total possible outcomes = 60.
1) Case 1. Ball is green. Here "favourable outcome" is getting a green ball.
No. of possible favourable outcomes = 16 (since there are 16 green balls)
So probability of getting a green ball
= 16/60 (from eqn(1))
= 4/15
2) Case 2. Ball is not green or red i.e. the "favourable outcome" is
getting a yellow ball (one that is not green or red)
No. of possible favourable outcomes = 24(since there are 24 yellow balls)
So probability of getting a yellow ball
= 24/60 (from eqn(1))
= 2/5
Hope you got it:)