how many pounds of coffee worth 10 dollars a pound should be added to 20 pounds of coffee worth 4 dollars a pound to get a mixture worth 5 dollars a pound
10x + 4*20 = 5(20 +x)
10x + 80 = 100 + 5x
10x - 5x = 100 - 80
5x = 20
x = 20 / 5 = 4 lbs
