2/3 of a milk-water mixture was milk. There was 21 lit of the mixture. If 4 lit of water is added to it, the % of milk in the new mixture will be:

Step 1: Understand the original composition

• Total mixture = 21 liters
• Milk = 2/3 of 21 = 14 liters
• Water = 1/3 of 21 = 7 liters

Step 2: Add 4 liters of water

• New water = 7 + 4 = 11 liters
• Milk remains unchanged = 14 liters
• New total mixture = 14 + 11 = 25 liters

Step 3: Calculate percentage of milk

% of milk = (Milk / Total mixture) × 100  
          = (14 / 25) × 100  
          = 56%

Answer: 56%

After adding 4 liters of water, the percentage of milk in the new mixture is 56%.

This question is a classic example of how basic ratio concepts and percentage calculations apply in mixture and solution problems. It's especially helpful for students preparing for competitive exams or interviews where quantitative aptitude is tested.