Paul sent in a solution for this problem:

Seonaid and Brid from St Andrew's explained how they knew how many pairs of legs each creature has:

We worked out how many pairs of legs these creatures had by dividing the number of legs by two because there are two legs in a pair.  

Another solution is as follows:- 

How many legs does each of these three creatures have?

Cat 2, Spider 8, Bird 2, 

How many pairs is that?

Cat 1, Spider 8, Bird 1, 

How many legs does each of these have?

Snake 0, Insect 6, Crab 8

How many pairs is that?

Snake 0, Insect 3, Crab 4