Question

a) For a 20-sided die, what is the probability that you will roll a 19 on the 3rd roll of the die? [2]
b) What is the expected waiting time until a 19 is rolled? [2]

Answer 1

@ParthKohli

Answer 2

okay so there are 20 sides
you want a 19
you have a 1/20 chance of getting a 19 on any throw
so you have 19/20 chance of failure
you wanna do
failure x failure x success
19/20 x 19/20 x 1/20

Answer 3

i can not answer part b sorry

Answer 4

the expected number of times is the inverse of the probability
20 rolls

Answer 5

1/p

1/1/20=20

is this right @UnkleRhaukus

Answer 6

note the expectation values is not always useful in making predictions

it is not likely it will take 20 rolls , but on average for all time 20 is expected

Answer 7

@Daenio Hey, the guys have misunderstood the question

Answer 8

Each roll is independent. Well, the probability of getting 19 on ANY roll is 1/20.
Answer to a is 1/20.

Answer 9

And, we have to wait till 20 turns to get a 19, theoretically.

Answer 10

but would expect ever number to take twenty turns?
would you roll a zero fo 19 turns and then on the last turn you roll a nineteen and all the other numbers

Answer 11

It's alright amor. Thank you guys for at least trying! :D