For this case we have the following data:
700 rabbits that represent 500% of the population of last year. So, to obtain the rabbit population of last year, we developed a rule of three:
700 -----------> 500%
x ---------------> 100%
Where x represents the rabbit population of last year.
Clearing x we have:

rabbits
In this way, last year there was a population of 140 rabbits.
Answer:
140 rabbits