Question

Bill wants to see how many times he needs to roll a standard number cube before getting a 5. What is the probability that Bill will roll a 5 on his third roll and not before?

a. 125/1296
b. 25/216
c. 1/6
d. 1/2

Answer 1

5/6 • 5/6 • 1/6 = 25/216 the answer is b

Answer 2

The answer is b. 25/216

Explanation:

Bill wants to see how many times he needs to roll a standard number cube before getting a 5.

p =
`(1)/(6)` (on an average: roll 6 times before getting a 5)

The required probability can be found as :

`(1)/(5)((5)/(6)* (5)/(6)* (5)/(6))`

=
`(5)/(6)* (5)/(6)* (1)/(6)`

=
`(25)/(216)`

So, the answer is b. 25/216