The problem states the ratio of yellow-eyed rats to grey-eyed hamsters as 5:72, but the task is to determine the number of blue-eyed rats. This information seems incomplete or unrelated directly to the blue-eyed rats. Typically, problems like this would need additional information about either the total population or a direct relationship with blue-eyed rats. However, considering the correct answer given is 12, it might imply a direct allocation of numbers unrelated to the provided ratio.
Since there is no further detail on how to find blue-eyed rats from the ratio of yellow-eyed rats to grey-eyed hamsters, one might assume an error in question formulation. Typically in such situations, external data or clarifications would be needed to conclusively solve the problem. Given the constraints and information directly, 12 following the usual assumptions could merely be conjectural without the real data link.