Please help with step by step explanation. Thank you.

Largest Number Guessing Strategy. A contestant is presented with 100 business cards placed face down on a table. The cards have been presented to 100 people who have each written a real number on the side that is now face down. The game show host turns over the cards one by one. At each step, the contestant can declare that he thinks the number just revealed is the largest. What strategy should the contestant use to decide when to declare? What, roughly, is the probability that he or she finds the largest number?