Explanation:
y=kx
say that y is the original prince and x is the sale price
15=k(10)
15/10=k
3/2=k
you can check with the next problem to check if it is the same
25=k(20)
25/20=k
5/4=k
so you take 5/4-3/2=-1/4 so the constant is -1/4
Hope that helps :)