Let the capacity of the first tank be 5L, second be 4L, and the third be 5L.
In tank 1, we have
Amount of water = 5 × 5/6 = 25/6 L
Amount of milk = 5 – (25/6) = 5/6 L
In tank 2, we have
Amount of water = 4 × 3/8 = 3/2 L
Amount of milk = 4 – (3/2) = 5/2 L
In tank 3, we have
Amount of water = 5 × 7/12 = 35/12 L
Amount of milk = 5 – (35/12) = 25/12 L
In the final mixture, we have
Amount of water
=++= Amount of milk
=++= Hence, milk to water ratio = 65 : 103