# Thread: Please help me to solve a problem on conditional probability

1. ## Please help me to solve a problem on conditional probability

Please help me to solve a problem on conditional probability:
91 from 144 fishes in the river are polluted, only 6 fishes from these 144 are checked for pollution. What is the probability to find a polluted fish?

2. ## Re: Please help me to solve a problem on conditional probability

Not quite exactly linked to conditional probability (it may be involved if you think you checked the fish in a sequential manner)

Have you learned hypergeometric distribution?

3. ## Re: Please help me to solve a problem on conditional probability

The original question was:
Given 6 fishes from these 144 are checked for pollution, what is the probability to find a polluted fish?

4. ## Re: Please help me to solve a problem on conditional probability

First of all I suppose the question mean to find 1 or more (up to 6) polluted fishes in the sample.

The very first hint is that it will be easier if you consider the complementary event - you cannot find a polluted fish.

If you are not allowed to use those combinatoric coefficients and hypergeometric pmf, then you can use those "elementary" methods - consider you check the 6 fishes sequentially, and thus invoking the conditional probability.

Ask yourself the probability of getting a non-polluted fish in the first check; given the result of the previous checks, how would that affect the probability of the next check.

BTW TS rule states that you need to show efforts for all those kinds of assignment questions. I think I am on the brink of breaching the rule (or already bleaching that lol). So unless you show some efforts and asking more precise question, instead of throwing out a question with nothing else, I do not think I can further reply you more on this.

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