Given

• We want to mix water with honey to gain 20% by selling the mixture at the cost price of honey.
• Let's assume the cost price of 1 unit of honey is 100 (to make calculations simpler).
• If we mix water with honey, and the selling price remains the same as the cost price of honey, the new cost price will be 120 (100 + 20% of 100).

Now, let's calculate the ratio of honey to water in the new mixture:

• Honey: 100 (original cost price) * 1 unit
• Water: 20 (20% of 100) * 1 unit
• So, the ratio of water to honey is 20:100, which simplifies to 1:5.
• This means we need to add 1 part of water for every 5 parts of honey. So, 1/5 or 20% of water must be added.
• Therefore, the correct answer is (a) 20%.