In a class, 24 students read physics 21 read mathematics and 9 read both physics and mathematics. If each student read at least one of the two subjects, find the number of students in the class.
We can use the principle of inclusion and exclusion to solve this problem.
If 24 students read physics, and 9 of them also read mathematics, then the number of students who only read physics is 24 - 9 = 15.
Similarly, if 21 students read mathematics, and 9 of them also read physics, then the number of students who only read mathematics is 21 - 9 = 12.
Now we can use the formula:
Total number of students = Number who read only physics + Number who read only mathematics + Number who read both
Total number of students = 15 + 12 + 9 = 36
Therefore, there are 36 students in the class.