A large aquarium should contain 10,000 liters of water when it is filled correctly. It will overflow if it gets up to 12,000 liters. The fish will get sick if it gets down to 4,000 liters. The aquarium has an automatic system to help keep the correct water level. If the water level is too low, a faucet fills it. If the water level is too high, a drain opens.

One day, the system stops working correctly. The faucet starts to fill the aquarium at a rate of 30 liters per minute, and the drain opens at the same time, draining the water at a rate of 20 liters per minute.

1. Is the water level rising or falling? How do you know?
2. How long will it take until the tank starts overflowing or the fish get sick?

A different aquarium should contain 15,000 liters of water when filled correctly. It will overflow if it gets to 17,600 liters.

One day there is an accident, and the tank cracks in 4 places. Water flows out of each crack at a rate of liter per hour. An emergency pump can re-fill the tank at a rate of 2 liters per minute. How many minutes must the pump run to replace the water lost each hour?