Solve the Problem $\$ 20$ A Pound That She Wishes to Mix with 90 Pounds

Multiple Choice

Solve the problem.
-A merchant has coffee worth $\$ 20$ a pound that she wishes to mix with 90 pounds of coffee worth $\$ 90$ a pound to get a mixture that can be sold for $\$ 80$ a pound. How many pounds of the $\$ 20$ coffee should be used?

A) $7.5 \mathrm { lb }$
B) $52.5 \mathrm { lb }$
C) $105 \mathrm { lb }$
D) $15 \mathrm { lb }$