Bag contains 5 red marbles and 3 black marbles,
For atleast one of the three marbles drawn be black, if the first marble is red,
then the following situations are possible
Event E_{1} = Second marble is black and third is red
Event E_{2} = Second marble is black and third is also black (E_{2})
Event E_{3} = Second marble is red and third is black (E_{3})
Let Event R_{i} = drawing red marble in ith draw
Event B_{i }= drawing black marble in ith draw