In what proportion water must be added to spirit to gain 20% by selling it at the cost price?

Question: In what proportion water must be added to spirit to gain 20% by selling it at the cost price?

Solution:
The percentage gain is essentially the ratio of pure spirit to water in the diluted solution. This gain is made because the addition of water increases the volume without increasing the cost, allowing the adulterated spirit to be sold at the original price (the cost price for the pure spirit).
Since the selling price equals the cost price (CP) in this case, the 20% gain represents the proportion of water in the solution.

Therefore, the proportion of spirit to water is 100% : 20% or 5 : 1.

Hence, water should be added to spirit in a 1 ∶ 5 proportion to gain 20% by selling it at the cost price.