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

$\displaystyle \int_{10}^{20} \int_{x/2}^{x} \frac{20-x}{25x}\ dydx$

Q2) I drew the graph for limits of integration

For the marginal density of x I have:

$\displaystyle \int_{x/2}^{x} \frac{20-x}{25x}\ dy = \frac{20-x}{50}$

For the marginal density of y I have:

$\displaystyle \int_{10}^{2y} \frac{20-x}{25x}\ dx + \int_{y}^{20} \frac{20-x}{25x}\ dx$

May I ask if my bounds correct in calculating this?

Q3) I had:

$\displaystyle \frac{20-x}{25x} \times \frac{50}{20-x} = \frac{2}{x}$

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