Hello! Essentially you are trying to find a linear function. For example, a linear function is a function that increases or decreases in some type of relationship between each pair consistently. To answer your question, one example could look like this...
Lastbenchstudent basically nailed the linear equation but I could elaborate from his work and answer your question in a different way.
A = 3, B = 18
A = 2, B = 9
A = 1, B = 4.5
To explain my answer, I basically divided B every time A decreased. As A decreased by 1, B decreased by half the amount!