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