When traveling with the wind, it takes an airplane 3 hours to travel 1800 miles. It takes the same airplane 36 hours to travel the same 1800 miles when traveling against the wind. Have 2 grades of milk, one that is 24% and one that is 18%. How much of each should they use to end with 42 liters of 20%?
a) 12 liters of 24% and 30 liters of 18%
b) 18 liters of 24% and 24 liters of 18%
c) 24 liters of 24% and 18 liters of 18%
d) 30 liters of 24% and 12 liters of 18%