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At first glance it may seem surprising that the two given distances fix the length of the pool. This context will also stimulate visualisation, as students 'picture' the motion of each swimmer and then watch for the moments when they cross.
Why wasn't the second crossing in the same place as the first?
If the swimmers can keep this going indefinitely will they have eventually had a crossing point everywhere along the length of the pool?
What happens if they both swim twice as fast?
What would happen if the pool was twice as long?