Solve the Problem $\$ 40$ A Pound That She Wishes to Mix with 80 Pounds

Multiple Choice

Solve the problem.
-A merchant has coffee worth $\$ 40$ a pound that she wishes to mix with 80 pounds of coffee worth $\$$ 70 a pound to get a mixture that can be sold for $\$ 50$ a pound. How many pounds of the $\$ 40$ coffee should be used?

A) $80 \mathrm{lb}$
B) $240 \mathrm{lb}$
C) $120 \mathrm{lb}$
D) $160 \mathrm{lb}$