How do you find the the constant of proportionality

Question

When Y=20, X=12 solve

Answer 1

if x and y vary directly then y=kx

and

20= k(12)

k= 20/12 = 5/3 = the constant of proportionality

but if x and y vary inversely then y=x/k

and

20= 12/k

k = 12/20 = 3/5 = the constant of proportionality