There are three vessels A, B and C having milk to water ratio as 3 : 5, 1 : 3 and 7 : y.  After adding mixture of vessel B to A, the milk to water ratio becomes 1 : 2 in A. Now 60% of mixture is taken out from vessel A and mixture of vessel C is mixed with remaining mixture of vessel A which results in a final milk to water ratio of 3 : 4. Find the value of y given that volume of B is 20 litres and volume of C is (7 + y) liters