Solve the Problem $20 \%$ Alcohol Solution Must Be Mixed with 90 Liters of A Alcohol

Multiple Choice

Solve the problem.
-How many liters of a $20 \%$ alcohol solution must be mixed with 90 liters of a $80 \%$ solution to get a $40 \%$ solution?

A) $27 \mathrm {~L}$
B) $18 \mathrm {~L}$
C) $180 \mathrm {~L}$
D) $270 \mathrm {~L}$