Q. What percent of water must be mixed with honey so as to gain 20% by selling the mixture at the cost price of honey?

Explanation : 

Percent of water must be mixed with honey

Let’s approach this step-by-step:

1) Let’s say we start with 100 units of honey.

2) We want to add water to this honey and sell the mixture at the same price as 100 units of pure honey, but make a 20% profit.

3) This means our mixture should be equivalent to 120 units of honey in terms of value (100 + 20% of 100).

4) But we’re selling it at the price of 100 units of honey. So our mixture must contain 100 units of honey.

5) If our mixture contains 100 units of honey and is equivalent to 120 units in value, the total mixture must be 120 units.

6) So we added 20 units of water to 100 units of honey.

7) To calculate the percentage:
(20 / 120) * 100 = 16.67% water in the final mixture

8) However, the question asks for the percent of water mixed with honey, not the percent in the final mixture.
So we calculate: (20 / 100) * 100 = 20%

Therefore, the correct answer is indeed a) 20%.