A fair die is thrown 20 times. The probability that on the 10th throw, the fourth six appears is a.`"^20 C_(10)xx5^6//6^(20)` b. `120xx5^7//6^(10)` c. `84xx5^6//6^(10)` d. none of these

Explanation

Let `n` be the total number of tosses.


Let `X` be the number of times head occurs


`Rightarrow P(event) = \ ^9 C_3 (1/6)^3 (5/6)^6 (1/6)`


`Rightarrow P(event) = 84 x (5^6 / 6^10)`