An urn contains 5 red and 5 black balls. A ball is drawn at random, its colour is noted and is returned to the urn. Moreover, 2 additional balls of the colour drawn are put in the urn and then a balls is drawn at random. What is the probability that the second ball is red ?

Answer

` = (1)/(2) xx (7)/(12) + (1)/(2) xx (5)/(12) = (1)/(2) ((7)/(12)xx(5)/(12))=(1)/(2)xx1=(1)/(2)`