Combination

A gold, a silver, and a bronze medal are awarded in an Olympic event. In how many possible ways can the medals be awarded for a 200-meter sprint in which there are 9 runners?

A computer system can be configured using 1 of 3 processors of different speeds, 1 of 4 disk drives of different sizes, 1 of 3 monitors, and 1 of 2 graphics cards. How many different computer systems are possible?

A professor gives his students 9 essay questions to prepare for an exam. Only 2 of the questions will actually appear on the exam. How many different exams are possible?

A group consists of 13 students (8 boys and 5 girls). Only 5 of them will be taken to a field trip. How many different selections of students exist, if it needs to contain at most 2 boys?

How many 3 or 4 digit codes could David create, if only odd digits are available for him to choose from, without any repeating?