So I will be given 5 waves even if I take 99 place in leaderboard, am I right?

Yes, you are absolutely right. But to take 99 place is not so easy

You need to play 30 minutes to take 99 place and get $ 7.

30 minutes? That much? I could do it in 15 minutes.

It depends on the place you get in each race...
You can't claim you'd do that in 15 minutes because it doesn't depend on you