I just answered your other question

http://www.mathhelpforum.com/math-he...s-problem.html

Have a look at that and see if it makes sense to you. Yes you're correct that the forces will be equal and opposite. There's one trick to this one. It gives you weight forces in Newtons. You'll need to convert those to mass before using the formula -

http://img508.imageshack.us/img508/1192/gravco8.jpg