Two balls are drawn at random with replacement from a box containing 12 black and 9 red balls. Find the probability that first ball is black and second is red.

$\frac{12}{49}$

Black ball in bag = 12

total ball in bag = 21

  probability that first ball is black =$\frac{12}{21}$ =$\frac{4}{7}$

  Ball has been replaced.

Now, red balls in bag = 9

 total balls in bag = 21

  probability that second ball is red =   $\frac{9}{21}$ =$\frac{3}{7}$

Probability of getting first ball black and second red = $\frac{4}{7}$ × $\frac{3}{7}$

= $\frac{12}{49}$