The constant of proportionality is the unit rate of something. So you know that if it's 6 bread pieces for every 8 ducks, you need to know how many ducks for how many pieces of bread. You would do 6 divided by 2 = 3 and 8 divided by 2 = 4. So 3 is the top number, and that is the constant of proportionality.
I hope I helped! :-)