In a chemical factory, a storage tank contains a mixture of alcohol and benzene mixed in the ratio 5 : 7. If 9 litres of the mixture is replaced by 9 litres of benzene, then the ratio of the two liquids becomes 5 : 8. How much alcohol was there in the tank?

Let alcohol = 5x and benzene = 7x. Total = 12x. Removing 9 litres removes 5/12 * 9 = 3.75 litres of alcohol and 5.25 litres of benzene. Remaining: Alcohol = 5x - 3.75, Benzene = 7x - 5.25 + 9 = 7x + 3.75. Ratio (5x - 3.75) / (7x + 3.75) = 5/8. Solving: 40x - 30 = 35x + 18.75, so 5x = 48.75. Initial alcohol was 5x = 48.75.