Your query seems incomplete. It looks like you want to know how much water must be added to 100 cc of an 80% solution to achieve a certain concentration, but the target concentration is missing. Could you please clarify the following:

• What is the concentration of the solution you want to achieve after adding water? (e.g., 50%, 20%, etc.)
• What is the substance or solution you are diluting? (optional, but helpful)

Once you provide the target concentration, I can help you calculate the amount of water to add.