A seedman has seeds worth 50 cents per pound mixed with seeds worth 80 cents per pound. If he wishes to make a mixture of 30 pounds to sell at 75 cents per pound, how many pounds of each should he use?

Let "e" represent the number of pounds of "expensive" seed (80¢/lb). The total cost of the seed can be written as
  (30-e)(.50) + (e)(.80) = 30(.75)
  30(.50) + e(.30) = 30(.75)
  e(.30) = 30(.75 - .50)
  e = 25
25 pounds of the "expensive" seed should be used, along with 5 pounds of the "cheap" seed.