729 ml of a mixture contains milk and water in the ratio

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729 ml of a mixture contains milk and water in the ratio 7 : 2. How much more water is to be added to get a new mixture containing milk and water in the ratio of 7 :3 ?

1. 60 ml
2. 70 ml
3. 81 ml
4. 90 ml

Correct Option: C

Milk = (729 x 7/9) = 567 ml
Water = (729 x 2/9) = 162 ml

Let water to be added is w
∵ 567 / (162 + w) = 7/3
⇒ 3 x 567 - 7 x 162 = 7 x w
⇒ 1701 - 1134 = 7 x w
∴ 7w = 1701 - 1134
∴ w = 567/7 = 81 ml