A container initially held a mixture of salt and water. After 20 litres of water were added, the concentration of salt in the resulting solution became 50%. Then, 60 litres of salt were further added to this solution, increasing the salt concentration to 75%. What was the original ratio of water to salt?

After adding 20 liters of water, the solution has 50% salt, meaning a total volume of V contains 0.5V salt. Adding 60 liters of salt brings the concentration to 75%, so 0.5V + 60 = 0.75(V + 60). Solving this gives V = 60 liters, meaning there was originally 30 liters of salt and 10 liters of water. The original ratio of water to salt is 10 : 30, which simplifies to 1 : 3.

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