I also need a lot of help with this subject along with upstream/downstream problems. Try this site: www.themathpage.com and then press mixture problems.
How Do You Do A Mixture Math Problems?
Mixture problems can be done by creating a mixture from two or more things, and then determining some quality like price or % etc of the resulting mixture. For example
This site provides you examples solving mixture problem >> www.purplemath.com
This site provides you examples solving mixture problem >> www.purplemath.com