15,20,25,30 and then there is 3,4,5,6. Find the constant proportionality

Cans of paint (k) = Birdhouses painted
x= cans of paint
y= Birdhouses painted
y= kx
Where k is the constant of proportionality.
Replace with a set of points and solve for k
(x,y) = (3,15)
15= k 3
15/3 = k
k=5