Your #5 solution is already correct.
A linear function has the form y=mx+b
you have noticed that y increases by 5 for every 10 steps along x. Therefore, your slope m is 5/10=1/2.
b is your y value at x=0
so y=(1/2)x+5
Additionally, you can calculate it with 2 points
using (10, 10) and (20, 15) and the elimination method just inserting x and y into the basic form
I:10=10m+b
II:15=20m+b
subtraction of II from I:
-5=-10m
-1=-2m
->m =1/2
insert this in I again:
10=10*(1/2)+b
10=5+b
5=b
again y=(1/2)x+5
