80 liters of orange juice must be mixed.
Step-by-step explanation
Lets assume, the amount of orange juice is
liters.
As the amount of mixture is 200 liters, so the amount of grapefruit juice will be:

Price of orange juice = 42¢ /liter , price of grapefruit juice = 32¢ /liter and the price of mixture = 36¢ /liter
So, total price of
liters orange juice
¢ ,
total price of
liters grapefruit juice
¢ and
total price of 200 liters mixture
¢
Thus the equation will be ....

So, the amount of orange juice to be mixed is 80 liters.