How Many Liters of A $30 \%$ - Alcohol Solution Must Be Mixed with 40 Liters of Must

Multiple Choice

How many liters of a $30 \%$ - alcohol solution must be mixed with 40 liters of a solution that is $60 \%$ alcohol to get a solution that is $50 \%$ alcohol?

A) $6 \mathrm{~L}$
B) $2 \mathrm{~L}$
C) $60 \mathrm{~L}$
D) $20 \mathrm{~L}$