The probability that a child will have a problem with alcohol is .75 if at least one of the parents is an alcoholic and .05 if neither parent is an alcoholic. In a recent study of a large number of children, 5% of the children involved had at least one parent who is an alcoholic. If a child in this study has an alcohol-related problem, what is the probability that at least one of the parents of the child is an alcoholic?

Statistics isn't my specialty, so I apologize in advance if I get the terms wrong.

What you need to do is find out how many children in the study have an alcohol related issue and have one alcoholic parent and divide it by the number of total children that have an alcohol related issue. So .75 of five percent is .75 \cdot .05 = .0357 or 3.57% have an alcohol related issue and have one alcoholic parent. The number of children that have an alcohol related issue and neither parent is an alcoholic is .05 of 95% .05 \cdot .95 = .0475 or 4.75%. So the total number of children with alcohol related issues is .0357 + .0475 = .0832 or 8.32%. So the probability that a child in the study with alcohol related issues will have one parent that is an alcoholic is \frac{.0357}{.0832} =.42908654 . When I say the number of children, I don't technically mean the number of children, I mean the proportion to the whole provided a sufficiently large sample space.

I hope this helps.