The ratio of total students in Ms. Murry's class to students in the class who have an older brother is 3 to 1. The answer is 1:2. What is the question?

Question

We can not figure out how to get the answer. 

Answer 1

What is the ratio of students in the class who don't have an older brother to students in the class with an older brother.

 

If there are 3 students in the class and only 1 student has an older brother, it can be inferred that the remaining 2 students don't have an older brother.