A class of 25 students includes the Smith quadruplets. The teacher must select two students from the class to represent the class in the school's student government. Instead of holding an election, the teacher decides to choose the two representatives at random. What is the probability that at least one representative will be a Smith?
You can choose 2 students out of 25 in: 25C2 = 300 ways - or 300 pairs.
Since each student can be paired with 24 other students and the 4 Smith brothers can form a total of: 24 + 23 + 22 + 21=90 pairs, therefore the probability that one of the pairs will be a Smith is: 90 / 300 =3 / 10