A merchant has coffee worth $70 a pound that she wishes to mix with 40 pounds of coffee worth $90 a pound to get a mixture that can be sold for $80 a pound. How many pounds of the $70 coffee should be used?

Answer: 40 pounds

Step-by-step explanation:

Let assume that the first coffee = F worth $70 a pound. That is 70F

to mix with the second coffee S 40 pounds of coffee worth $90 a pound. That is 40 * 90 = 3600

to get a mixture M that can be sold for $80 a pound. That is 80 (40 + F)

M = F + S

80 (40 + F) = 3600 + 70F

3200 + 80F = 3600 + 70F

10F = 400

F = 40 pound