A box has 5 blue and 4 red balls. One balls is drawn at random and not replaced. Its colour is also not noted. Then another ball is drawn at random. What is the probability of second ball being blue?

Explanation

Let `E_1`​ be the event that first ball drawn is blue and

`E_2​` be the event that first ball drawn is red and E is the event that second ball drawn is blue.

So,


`P(E)=P(E_1​)P(E/(E_1)​)+P(E_2​)P(E/(E_2)​)`


`=5/9​.4/8​+4/9​.5/8​=(40)/(72)​=5/9`​