Thanks to all who sent in solutions. Here are some from Oakley School and St. George's school. This is what Emily sent us;

If you TAKE 1 rat you have to ADD 2 people
E.G
2 people + 196 rats 98 people + 101 rats
4 people + 195 rats 96 people + 102 rats
6 people + 194 rats
8 people + 193 rats
10 people + 192 rats
etc.
Hope this helps!

Ben, Hannah & Scott sent in these thoughts;

We figured that if you add 1 rat and take 2 people. For example 108 rats and 84 people, 109 rats and 82 people and 110 rats and 80 people.

Otis, Jason & Sophis sent their solutions for 600 legs;

our solutions are:
298 people, 1 rat
298x2 = 596 legs
1 rat = 4 legs
298 people(596 legs) + 1 rat(4 legs)

150x2 = 300 legs
75x4 = 300 legs
150 people(300 legs) + 75 rats(300 legs)