Two balls are drawn at random with replacement from a box containing 7 black and 4 red balls. Find the probability that first ball is black and second is red.

28/121

Total number of balls = 11

number of black balls  =7

number of red balls  = 4

probability of getting first ball black = 7/11

the ball is replaced after the first draw.

So, probability of getting second ball as red = 4/11

Therefore, the probability of getting first ball as black and second ball as red = (7/11) x (4/11) 

                                                                                                                = 28/121