Look at the table for each type of bird. 5x=15. 6x=18. 10x=30. What would you multiply each of those by to get your answer and find the difference? 5x=15; x= 3. It is the same for each number. So the difference is by 3. I think if I understand this correctly, that would be how you do it. Does this help?
Sample answer: Divide the number of robins by the number of cardinals for each pair of values. The ratios are the same and are a constant of proportionality of 3.
