First:
Assume Earth's population is 8,037,860,611 and they have ID numbers 1 through 8,037,860,611
You make a set of cards numbered 1 through 8,037,860,611
A robot goes to each person in turn and shows them a card at random (cards get replaced)
If anyone is shown the number that matches their ID, I will GAIN $8,037,860,611 (one time only)
Second:
You pick a population size n, and these people get new ID numbers 1 through n
You make a set of cards numbered 1 through n
This time the robot gives each person a card, such that everyone has a unique card
If anyone has the card that matches their ID number, I will LOSE $8,037,860,611 (one time only)
The Question:
Assume your goal is to make me break even as much as is possible after doing these two things. What population size, n, do you choose and what is my expected value to the nearest cent?
You can have a perfect understanding of the formulas involved and still get the wrong answer. This question involves big numbers and high precision... something typical calculators have trouble with. You might have to additionally prove your methods or calculation tool don't run into this pitfall