From the 50 liters of milk, 5 liters of milk is taken out and after it 5 liters of water is added to the rest amount of milk. Again 5 liters of milk and water is drawn out and it was replaced by 5 liters of water. If this process is continued similarly for the third time, the amount of milk left after the third replacement:

• A. 45L
• B. 36.45L
• C. 42.5L
• D. 40.5L

Answer: (B)

General Formula: Final or reduced concentration = initial concentration x  1 - a m o u n t   b e i n g   r e p l a c e d   i n   e a c h   o p e r a t i o n t o t a l   a m o u n t n where n is the number of times the same operation is being repeated. The "amount being replaced" could be pure or mixture as per the case. similarly ,"total amount" could also be either pure or mixture. Here amount being replaced denotes the quantity which is to be withdrawn in each time.   Therefore,     50 × 1 - 5 50 3 = 36.45 L