A bag contains 20 balls. 8 balls are green, 7 are white

Solution

A bag contains 20 balls Calculation: ⇒ We have to at least one ball of each colour ⇒ Even if we pick 8 balls there is a probability all of them may be green. Hence we must pick more than 8 balls for sure ⇒ Now even we pick (8 + 7) balls or 15 balls, there is a  probability that all of them may be green and white only with no red ball picked at all. ⇒ Hence taking the worst possible scenario in consideration we must pick = 15 + 1 = 16 ∴ The required result will be 16.