Why do this problem?
is a good way to increase familiarity with negative numbers on a number line. In answering the questions, children begin to calculate with negative numbers in a context that will give them confidence rather one which they perceive as difficult.
It would be a good idea to print out the picture so that children can put on the numbers and more easily use a ruler to see which objects are at a certain level. To make the link with calculation more explicit, you could have a go at writing number sentences for some of the questions as a class, then challenge the children to complete number sentences for the other questions. Pupils can then
go on to invent questions of their own and, in particular, they could find alternative ways of asking the same question. For example:
What depth is the octopus?
What number will the octopus be at?
How far from the surface is the octopus?
Tell me about the depth of these fish.
How do you know? How are you working this out?
You could invite children to make up as many questions as they can which have, for example, the answer $3$m. This allows them to be as creative as they like, and is a good way to assess their understanding.
Some children will benefit from having a go at the Swimming Pool
problem before this one.