The ratio of juice to water in container B is 4 : 3, respectively. 21 liters of the mixture is taken out and poured into an empty container C. If 4 liters of pure juice are added to container C, then find the difference between the quantities of juice and water in container C (in liters).

The correct answer is 7.

Step 1: Understand the ratio in container B
The ratio of juice to water in container B is given as 4 : 3.
This means that for every 7 parts of the mixture, 4 parts are juice and 3 parts are water.

Step 2: Calculate the quantities of juice and water removed from container B
When 21 liters of the mixture is taken out of container B, the ratio of juice and water in the removed mixture remains 4 : 3.

Total ratio parts = 4 + 3 = 7 parts.

Quantity of juice taken out = $\frac{4}{7}\times 21$ = 12 liters.

Quantity of water taken out = $\frac{3}{7}\times 21$ = 9 liters.

Step 3: Determine the initial quantities in container C
Since this 21 liters of mixture is poured into an empty container C:
- Initial quantity of juice in container C = 12 liters
- Initial quantity of water in container C = 9 liters

Step 4: Account for the added pure juice
Now, 4 liters of pure juice are added to container C.

New quantity of juice in container C = 12 + 4 = 16 liters.
Quantity of water in container C remains = 9 liters.

Step 5: Find the difference between the quantities of juice and water in container C
Difference = Quantity of juice - Quantity of water

Difference = 16 - 9 = 7 liters.

Thus, the difference between the quantities of juice and water in container C is 7 liters.