Rajesh purchased two different kinds of alcohol. In the first mixture, the ratio of alcohol to water is 4 : 5 and in second mixture it is 6 : 7. If he mixes the two given mixtures and makes a third mixture of 22 litres in which the ratio of alcohol to water is 5 : 6, the quantity of first mixture that is required to make 22 litres of the third kind of mixture is.

How many kg of rice costing ₹42 per kg should be mixed with$$7{1\over 2}$$ kg rice costing ₹50 per kg so that by selling the mixture at₹ 53.10 per kg, there is a gain of 18%?