A strange scene : imagine a conveyer belt, with tins placed at regular intervals onto the belt at one end, moving along at a steady rate towards a labelling machine at the far end. A gerbil starts from a tin at the beginning of the belt and steadily jumps from tin to tin, finally jumping off immediately before the labelling machine.

When the gerbil jumps at a rate of one jump per second he arrives at the labelling machine in 20 jumps but if he speeds up and manages two jumps per second he can fit in 32 jumps before the end of the line.

How many tins are there on the line ?