Statistics Problem.

Greetings, I was solving a probability exercise from a Stats pastpaper and I got stuck in 1 mark question.

Erika is a birdwatcher. The probability that she will see a woodpecker on any given day is 1/8. It is assumed that this probability is unaffected by whether she has seen a woodpecker on any other day.

i) Calculate the probability that Erika first sees a woodpecker

(a) on the third day [3] - I got 49/512 which is right

(b) after the third day [3] I got 343/512 which is right

ii) Find the expectation of the number of days up to and including the first day on which she sees woodpecker [1]

I don't know how to calculate E(X) without knowing how many times she will go. Or maybe they're asking for something else?

Reply 1

zed963

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Original post by Blackfyre

Greetings, I was solving a probability exercise from a Stats pastpaper and I got stuck in 1 mark question.

Erika is a birdwatcher. The probability that she will see a woodpecker on any given day is 1/8. It is assumed that this probability is unaffected by whether she has seen a woodpecker on any other day.

i) Calculate the probability that Erika first sees a woodpecker

(a) on the third day [3] - I got 49/512 which is right

(b) after the third day [3] I got 343/512 which is right

ii) Find the expectation of the number of days up to and including the first day on which she sees woodpecker [1]

I don't know how to calculate E(X) without knowing how many times she will go. Or maybe they're asking for something else?