X is going up by +1, until you get to (n)
Y is going up by +2, until you get to 20
So, we see that when x=5 and y=10, to get there, you do 5 × 2 = 10, and we look at all of the other (x's and y's) and see that when you multiply (x) by (2) you get y.
So, our rule would essentially be: x × 2 = y
So, if y=20, then you do 20 ÷ 2, which equals 10.
So, n=10
~Hope this helped!~