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A polling agency would like to investigate the relationship between people having seen the movie Jurassic World and people having nightmares. In A, 37% of the population has seen the movie. We also know that in general, 19% of the people that have seen the movie have nightmares afterwards. How much people in A should the polling agency expect to question before coming across the first person that has seen the movie and that has had nightmares afterwards ?

I really suck at probability. Here is what I did

P(in A)= 0,37
P(movie and nightmare)= 0,19
P(in A movie and nightmare)= 0,19 * 0,37 =0,0703

I don't know how to answer how many people one should expect to question before......

I don't even see where to begin :( :(

Can someone help me please???

Thank you.

alison monroe
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You simply take its inverse to figure out how many people you can expect.

That is, you can expect that it will take $\frac{1}{P} = \frac{1}{0.0703} \approx 14$ people to run into one (so maybe that translates into having to question $13$ before you run into one)

The math behind this is a bit technical, but this is a well-known formula. E.g. You can expect to have to roll a die $6$ times before you get the first $3$, as the chance of getting a $3$ is $\frac{1}{6}$. Sometimes you have to roll the dice fewer than 6 times, and sometimes more than 6 times to get the first $3$, but on average it turns out it takes $6$ rolls. Here is a question with some answers that try and make some sense as to why this is so.

Bram28
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  • @alisonmonroe You're welcome! I added a link where you see some of the math that explains why it is $\frac{1}{P}$ – Bram28 Jul 27 '17 at 18:22