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?
Choose the correct choice from the following answers.
A) 3
B) 4
C) 5
D) 6
E) 7
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 7.
<result>E</result>
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?