May I obtain critiques/hints/tips on my approach?

Q1) I evaluated the double integral and it was equal to 1. I also substituted x=10, 20 into the function and confirmed it is >0

Q2) I drew the graph for limits of integration

For the marginal density of x I have:

For the marginal density of y I have:

May I ask if my bounds correct in calculating this?

Q3) I had:

Q4) I feel it is related to Q3, and wanted to see if I have Q3 correct first