Vijay purchased two different kinds of alcohol. In the first mixture, the ratio of alcohol to water is 3 : 4 and the second mixture it is 5 : 6. If he mixes, the two given mixtures and makes a third mixture of 18 litres in which the ratio of alcohol to water is 4 : 5, the quantity of the first mixture (whose ratio is 3 : 4) that is required to make 18 litres of the third kind of mixture is: