Bonus Question on Extra Credit work... 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!!