Original quantity of water in 729ml mixture =29×729=162ml
Let the x ltr. of water is added to form the milk and water ratio as 7:3
Equating amount of water after addiction
So 310(729+x)=162+x
⇒2187+3x=1620+10x
⇒567=7x⇒x=81ml
Hence, option B is the correct answer.