So what you want to do here is get the fractions to have the same denominator.
To do this, you first want to get rid of the 2 from each number.
You can just subtract 2 from each because that won't truly impact the difference, except for if they were different, as in something like 3 2/5 and 4 2/5
So you're left with the fractions 3/8 and 2/5.
Now to get them to have the same denominator, you first find the LCM, or least common multiple. This can be found by just multiplying 8 and 5 together.
8 * 5 = 40
So what you can say is:
3/8 * 5/5 = ?
Since 5/5 = 1.
3/8 * 5/5 = 15/40
2/5 * 8/8 = 16/40
So if you compare the two fractions, you can tell that your friend's skateboard ramp is slightly higher.