The excess weight is zero!

According to Archimedes' principle, a floating body displaces a volume of liquid with the same weight as its own. When the ship enters the trough, therefore, $c$ tons of water have flowed out and the weight of the trough with its contents has thus remained unchanged.
## Task 22/61

The trough of a ship lift weighs $a$ t in its empty state, and the water filling weighs $b$ t. To balance, counterweights with a total weight of $(a+b)$ t are attached.

How great is the excess weight of the trough with its contents if a ship with a weight (including cargo) of $c \mathrm{t}$ is in the trough?