Total mixture=729 liters
Amount of milk in mixture =79×729=567liters
Amount of water in mixture =29×729=162liters
Let x liter is water is added to get the ratio 7:3 then
⇒567162+x=73
⇒1701=1134+7x
⇒7x=567 ⇒x=81liters
Hence 81 liters water is added.