If I bring $20, I pay $10 at the first arena which leaves me $10. When inside it is doubled to $20. Then I pay $10 to get out. I have $10 left.
The same happens at each arena. Finally I have $10 left over.
The experience cost me $10 in all.
Actually, though it rained 40 days and 40 nights, it was Noah that was on the ark a whole year.
And the number of animals is found in Genesis 7:2 "Of every clean beast thou shalt take to thee by sevens, the male and his female: and of beasts that are not clean by two, the male and his female."