There are three bottles of mixture of syrup and water in the ratios 2 : 3, 3 : 4 and 7 : 5. 10 litres of the first and 21 litres of the second bottle are taken. How much quantity of mixture from the third bottle is to be taken so that the final syrup and water ratio of the mixture from the three bottles will be 1 : 1, if the final mixture from all three bottles is added and mixed in a big container?