How do I make it so the faster you go, the more in-game currency you get?

Basically you start the small obby, it times you, and you step on a button to stop the timer, how do I make it so that the faster you do the obby, the more currency you get

You need a ratio for the amount of time it takes. For the amount of time that durates, the payout declines, for the lesser amount of time that durates, the payout increases.