Let:

• C be the cost price per unit of honey.
• The amount of honey used is 100 units.

The total cost price of the honey is $C \times 100$.

Step 1: Selling Price Calculation

We want to sell the mixture at a 20% profit, so the selling price of the mixture should be: $1.20 \times C \times 100$

Step 2: Total Selling Price of Mixture

The total selling price is also given by: $(100 + W) \times C$ Where W is the amount of water.

Step 3: Set the Selling Prices Equal

$1.20 \times C \times 100 = (100 + W) \times C$

Step 4: Simplify the Equation

$1.20 \times 100 = 100 + W$

$120 = 100 + W$

Step 5: Solve for W

$W = 120 - 100 = 20$

So, the amount of water to mix is 20 units.

Step 6: Calculate the Percentage of Water

The percentage of water is: $(\frac{20}{100}) \times 100 = 20\%$

Conclusion: The percentage of water to mix is $20\%$.