A bag contains 3 white, 4 red and 5 black balls. Two balls are drawn one after the other, without replacement. What is the probability that one is white  and the other is black?

Explanation

Given :

box =(3 W+4 R+5 B) balls

P (one white and one black) =P (first white and second black) +P (first black and second white )

`=frac{3}{12} times frac{5}{11}+frac{5}{12} times frac{3}{11}`

`=frac{15}{132}+frac{15}{132}`

`=frac{30}{132}`

`=frac{5}{22}`