(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

There iskcarps in the pool,mof them are marked. I randomly fish outncarps and see thatxof them are marked.

What is the expected value of number of carps in the pool? (ie. expected value of number of the carps in the pool in the beginning)

How will the expected value change as we fish the carps out one by one? (ie. not together, but one carp at the time).

To sum it up, I knowm,nandxand I don't knowk.

2. The attempt at a solution

I'm not sure how to solve it. For the first case, I think that ratio of marked carps in the group of the carps drawn out should be (averagely) the same as the ratio or marked carps in the whole pool, ie.

[tex]

\frac{x}{n} = \frac{m}{k}

[/tex]

so

[tex]

k = \frac{m.n}{x}

[/tex]

But will it really determine the expected value of number of all carps?

For the second case, I'm even more out-of-idea.

From definition, I'd say expected value of number of the carps is

[tex]

\sum_{i=0}^{k} X_i p_i

[/tex]

Where [itex]X_i = i[/itex] and [itex]p_i[/itex] is probability that there are [itex]i[/itex] carps in total.

But this doesn't seem to be an effective approach.

Could someone give me some hint, please?

Thank you.

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Expected value of number of carps pool

**Physics Forums | Science Articles, Homework Help, Discussion**