Why do this problem:
This is certainly an exercise in 'visualisation' or rather an
exercise in working through various visualisations until one is
reached that does what is needed.
There are a number of ways to approach this problem but seeing it
as a line being consumed from both ends simultaneously is a
particularly helpful one.
Students may have to work quite hard just to find a first
visualisation and then continue their effort until they find a
visualisation that lets them 'see' how the problem may be solved.
Rates are a fundamental idea in mathematics and this problem offers
a challenging context in which to encounter and consider these
kinds of measure.
Possible approach :
For students who need a 'concrete' phase follow the idea in the
'Possible support' section below. For more able students the
questions below should provide enough of a prompt and plenty of
time should be allowed in which they can work towards a first
visualisation and its subsequent improvements. Using the prompt
questions in the 'Possible support' section too early will rob
students of the important opportunity to arrive at their own
visualisation.
For the very ablest students this problem provides a valuable
context in which they may gain confidence and 'feel' for this type
of problem.
Key questions :

What are we asked to find ?

What else might it be helpful to know so we can do that ?

Is there a way we might discover that ?
Possible extension :
Encourage abler students to see the connection between this context
and the
Swimmers
problem.
Possible support :
Set up a line of 'tins' (plastic cubes maybe, the number doesn't
matter but 20 maybe)
Have one student consuming as the gerbil and another student
consuming as a labelling machine from the other end.
Ask students what information is needed so that the gerbil and the
machine can eat their way along the line a second's worth at a
time.
This should establish a visualisation and the next step is probably
trial and improvement, adjusting the machine's rate and the number
of 'tins' until the students can engage with the values given in
the problem.
 How long did each run last ?
 How many more tins did the gerbil manage when it ran faster
?
 How much less consuming did the machine manage on the gerbil's
second run ?