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

A 405 ml

B 81 ml

C 72 ml

D 91 ml

Correct Answer: Option A

The quantity of milk in the mixture is

7/(7+2) X 729

= 7/9 X 729

= 567mL

The quantity of water then shall be

729 - 567 = 162mL

If the final mixture would contain half milk and half water, there must be equal quantity of both.

So in order to make up for water, we'll need to add

567mL - 162mL= 405mL of water