A rectangular tank 90cm long and 60cm wide is mounted on bearings so that it is free to turn on a longitudinal axis. The tank has a mass of 68kg and its centre of gravity is 15cm above the bottom....

A rectangular tank 90cm long and 60cm wide is mounted on bearings so that it is free to turn on a longitudinal axis. The tank has a mass of 68kg and its centre of gravity is 15cm above the bottom. When the tank is slowly filled with water it hangs in stable equilibrium until the depth of water is 45cm after which it becomes unstable. How far is the axis of the bearings above the bottom of the tank?