If a die is a plain die with uniform sides, the number of possible outcomes in the sample space is equal to 6 multiplied to itself for 20 times, 6²⁰. If exactly four 2's should be in the outcome of the experiment, the sample space will become,
(1⁴)(5²⁰⁻⁴) = 5¹⁶
Thus, the probability is equal to 5¹⁶/6²⁰.
