Question 840424
http://www.algebra.com/algebra/homework/word/mixtures/mixture-price-two-part-both-parts-


http://www.algebra.com/algebra/homework/word/mixtures/mixture-price-two-part-both-parts


The lesson is what you need.  (one of those addresses WILL work; not sure which).
The same general problem has been asked and answered many, many, many times.