Joint pdf given as kxy for 0 < x < y < 1.

Find the value of k.

I understand the process of finding k - doing the double integral and setting it to 1. What I dont understand is the limits of integration for y.

I've seen two different limits set, but I still cannot seem to figure out how and why it is done.

I have seen the integral of x from 0 to 1 and the integral of y from x to 1. I have also seen the integral of x from 0 to 1 and the integral of y from 0 to y. Both give the correct answer of k = 8. My question is how do you go about choosing the limits for the y integral?

Thanks a lot!