A customer in a coffee shop purchases a blend of two coffees: Kenyan, costing $3.50 a pound, and Sri Lankan, costing $5.60 a pound. He buys 7 lb of the blend, which costs him $33.95. How many pounds of each kind went into the mixture?

Let s represent the amount of the high-priced coffee. The total cost can be computed as
  5.60s + 3.50(7-s) = 33.95
  2.10s = 9.45    (subtract 24.50)
  s = 4.5    (divide by 2.1)

4.5 pounds of Sri Lankan were mixed with 2.5 pounds of Kenyan.