In a mixture of 30 litres of water and petrol, the amount of water is 37.5%. What amount of water should be added to the mixture so that the amount of water becomes 68.75%?

In the initial 30 litre mixture, water is 37.5 percent, which is 3/8, so the exact amount of water is 3/8 multiplied by 30, giving 11.25 litres. If we let x be the litres of water to add, the new water concentration becomes 68.75 percent, which is 11/16, leading to the equation (11.25 + x) divided by (30 + x) equals 11/16. Multiplying across gives 180 + 16x = 330 + 11x, so 5x equals 150, meaning the amount of water to add is 22.5 litres.

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