Cheng has a garden pond, which he tops up at the beginning of every week from a 200-liter rain barrel, which is in turn filled by rainwater from a section of his roof. At the beginning of summer both the pond and rain barrel are full. The average weekly summer rainfall where he lives is 5 mm. The section of his roof which collects rain for his barrel has an area of 6 square meters. He uses 60 liters of water per week to top up the pond.
For how many weeks will Cheng have enough water in the rain barrel to top up the pond fully?