A storage tank is being filled and emptied. The volume of water in the tank, in liters, at time t minutes is described as follows:
- At t=0 minutes, the volume is 100 liters.
- It increases linearly to 500 liters by t=10 minutes.
- It then decreases linearly to 200 liters by t=25 minutes.
- From t=25 minutes to t=35 minutes, the volume remains constant at 200 liters.
- Finally, it increases linearly to 600 liters by t=55 minutes.
What is the total duration, in minutes, during which the volume of water in the tank was increasing?
