A merchant has coffee worth $20 a pound that she wishes to mix with 50 pounds of coffee worth $80 a pound to get a mixture that can be sold for $60 a pound. How many pounds of the $20 coffee should be used?

Solution:

X pounds of the $20 coffee should be used

So

20x+50*80 = 60*(x+80)

x=25

So 25 pounds of the $20 coffee should be used