Solve the Investment Problem $\$ 5$ A Pound That She Wishes to Mix with 80 Pounds

Solve the investment problem.
-A merchant has coffee worth $\$ 5$ a pound that she wishes to mix with 80 pounds of coffee worth $\$ 8$ a pound to get a mixture worth $\$ 7$ a pound. How many pounds of the $\$ 5$ coffee should be used?

A) $40 \mathrm{lb}$
B) $60 \mathrm{lb}$
C) $20 \mathrm{lb}$
D) $120 \mathrm{lb}$