There are no rules but 65 feet makes the most sense
You touch a creature. The creature's jump distance is tripled until the spell ends.
when you make a running long jump, the distance you can cover increases by a number of feet equal to your Strength modifier
The bonus to jump should follow the order in which the bonuses are actually applied in the game. In other words, since you apply the spell before you jump, your jumping distance is tripled first. Then, when you actually do a running long jump, the remarkable athlete bonus gets added onto that.
Thus, the math follows the order in which the bonuses are actually applied.
Constant versus situational bonus
Another way to reach the same conclusion is to think of the two effects in terms of how general they are. While under the effects of jump, your jump distance is constantly tripled. However, remarkable athlete only gives you the bonus to jump when you are actually making a running long jump. If you make a standing long jump you don't get any bonus at all from remarkable athlete, but jump still applies.
I would argue the constant general effect gets applied, and then the situational bonus gets added on top of that.
Either way you think about it, the math would work out to:
(20 x 3) + 5 = 65 feet
Note regarding jumping and speed
The distance you jump is indeed limited by your speed but with dashing and other means of speed increases the jump distances can be kept relevant as indicated in this conversation with Jeremy Crawford:
Q: Can you jump farther than your movement when using magic i.e spell
Jump & boots of striding and springing?
A: To be clear, things like the jump spell don't increase speed. You
can jump crazy far, but your speed caps it.
Q: Are you saying you can't jump farther than your speed even with
Jump spell or Boots of Striding and Springing?
A: Every foot jumped costs movement, so you can jump farther than your
current speed if you take the Dash action.
This is discussed more here Does a ring of jumping let you jump farther than your normal movement? and What happens when you run out of movement while jumping?