There is a water tank with a capacity of 1200 litres and 10 pipes are attached to it. Some of these pipes fill the tank while others empty the tank. Each pipe can fill or empty the tank at the rate of 10 litres per minute. When all pipes are opened, a half filled tank gets completely filled in 30 minutes. How many pipes are there those empty the tank? 1. 2 2. 3 3. 4 4. 1 5. Cannot be determined