a certain drink of type a is prepared by mixing 4 parts milk with 3 parts fruit juice . another drink of type b is prepared by mixing 4 parts of fruit juice and 3 parts of milk . how many liters of fruit juice must be added to 21 liters of drink a to convert it to drink b ?
"in 21 liters of drink a , there are 12 liters of milk and 9 liters of juice . with 12 liters of milk , we need a total of 16 liters of juice to make drink b . we need to add 7 liters of juice . the answer is e ."
subtract(divide(multiply(multiply(divide(4, add(4, 3)), 21), 4), 3), multiply(divide(3, add(4, 3)), 21))