Question

1) A diary man pays Rs 6.40 per litre of milk. He adds water and sells the mixture at Rs 8 per litre, thereby making 37.5% profit. The proportion of water to milk received by the customers is:
(Plz explain how to do this)

Answer 1

The diary man makes 37.5%
To make a profit of 37.5% the selling price would have been=6.4*137.5/100
                                                                                               = 8.8 ruppees
But he has sold the milk after adding water at Rs 8
He has added water to create value of 8.8 rupees
Hence water % added is (8.8-8)100/8=10%
The ratio of water to milk 10/100=1:10

Answer 2

Profit made by dairy man is = Rs 37.5 * 6.40 /100 = Rs 2.40   per litre
       cost of water is zero - assumption.

  One litre of pure milk + x litres of water is sold at Rs 6.40 + Rs 2.40 = Rs 8.80
   that is, 1+x litres of mixture is fetches him Rs 8.80.

  Then, one litre of mixture is sold at Rs 8.80 / (1+x)   = Rs 8

                8.80 = 8 + x * 8
        x = 0.80/8 = 0.10
  
  So dairy man adds 100 ml of water to 1 litre of milk to get 1.1 litre of mixture.

   in one litre of mixture, we have 100/1.1 ml of water and  1000/1.1 ml of milk.

 ratio will be  100: 1000  =>  1 : 10