640 ml of a mixture contains milk and water in ratio 6:2. How much of the water is to be added to get a new mixture containing half milk and half water?

Step 1: Find initial milk and water quantity

• Total mixture = 640 ml
• Milk : Water = 3 : 1
• Milk = (3/4) × 640 = 480 ml
• Water = (1/4) × 640 = 160 ml

Step 2: Final desired ratio is 1:1 (milk = water)

• To make milk = water, we need to increase water to 480 ml
• Current water = 160 ml
• Required additional water = 480 - 160 = 320 ml

Answer: 320 ml

We must add 320 ml of water to the 640 ml mixture to make the ratio of milk to water 1:1.

This problem is a good example of how ratio adjustments can be achieved by keeping one quantity constant (milk in this case) and manipulating the other to balance the ratio. It’s a typical question seen in aptitude tests where precision with proportion is essential.