To solve this question, first look and see what are the variables the question is asking for. In this case, the question is asking for how many students ONLY have Rabbits and also how many students ONLY have birds.
Since the question is how many more rabbits are there than birds, you would need to find the difference which is 3.
25 - 22 = 3. 3 additional students have rabbits only compared to birds only.
