Dairyman would earn 25% profit, if only milk were sold at Rs 8 per liter.
Since the profit earned is 37.5% by selling milk and water, the additional 12.5% profit is due to selling only water.
12.5% of Rs 6.40 is 80p, which can buy 1/10th of a liter of milk from Diaryman.
Therefore, the proportion of water to milk received by the customers is 1:10.