A Level OCR Hard stats question

Matheen1

I have completed 15a and 15b but I cant get my head around what to do to solve part c?

Reply 1

old_engineer

Original post by Matheen1

I have completed 15a and 15b but I cant get my head around what to do to solve part c?

You can think of the scenario in part (c) being broken down in to two events

Event 1: X = 2 picked exactly two times in the first nine picks
Event 2: X = 2 picked at the tenth pick

Reply 2

Matheen1

Original post by old_engineer

You can think of the scenario in part (c) being broken down in to two events

Event 1: X = 2 picked exactly two times in the first nine picks
Event 2: X = 2 picked at the tenth pick

Can u explain why these two events are please? I just can’t get my head around it.

Reply 3

RDKGames

Study Forum Helper

Original post by Matheen1

Can u explain why these two events are please? I just can’t get my head around it.

She picks until she gets three 2s. If she stops on 10 picks that means the 10th pick must be the third 2.

The first two 2s can appear anywhere in the first 9 picks. No restriction on those.

(edited 10 months ago)

Reply 4

old_engineer

Original post by Matheen1

Can u explain why these two events are please? I just can’t get my head around it.

Freda chooses values of X until the number X = 2 has come up exactly three times. This could happen after just three picks, or it could happen after many many picks, or anything in between. The question asks us for the probability that it takes exactly ten picks to choose exactly three occurrences of X = 2. That means that the third X = 2 must come up at the tenth pick, which in turn means that exactly two times X = 2 must have come up during the first nine picks.