Twelve balls are distributed among three boxes. The probability that the first box contains three balls is

Since each ball can be placed in any one of the 3 boxes, therefore there are 3 ways in which a ball can be placed in any one of the three boxes. Thus, there are 312 ways in which 12 balls can be placed in 3 boxes. The number of ways in which 3 balls out of 12 can be put in the box is 12C3 The remaining 9 balls can be placed in 2 boxes in 29 ways. So, required probability is 12C331229=11092310