1. Without consultation with each other, each of four organizations announces a one-day convention to be held during June. Find the probability that at least two organizations specify the same day for their convention.
2. There were 16 presidents of the Continental Congress from 1774 to 1788. Each of the five students in a seminar in American history chooses one of these presidents on whom to do a report. If all presidents are equally likely to be chosen, calculate the probability that at least two students choose the same president?
3. Eight workers need an employee number to be printed on a name badge. The company's security office uses a random-number generator to assign each employee a number between 81 and 100, inclusive. What is the probability that at least two workers receive the same number?
4. Each person in a group of 10 people randomly selects a number from 1 to 100, inclusive. What is the probability that at least two people select the same number?