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How can I do this, where do I start? I haven't taken stats before so I'm just curious to see if someone could show me how:

A study considered risk factors for HIV infections among IV drug users. It found that 40% of users who had less than or equal to 100 injections per month (light users) and 55% of users who had greater than 100 injections per month (heavy users) were HIV positive.

Suppose we have a group of 10 light users and 10 heavy users.

What is the probability that exactly 3 of the 20 users are HIV positive?

I'm interested in the methodology and the solution if possible.

Thanks for your help!!

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# Statistics Probability - How do I do this?

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