In a class of 30 students, 20 take mathematics, 17 take physics and 2take none of the two subjects. How many students take both subjects?
Here is a Venn:
Of the 30 students, 2 do not take either subject; subtracting these 2 students leaves a total of 28 students.
Of these 17 take physics and 20 take math; adding these numbers together, we get 35.
Therefore, 35 - 28 = 7 students; these 7 students take both classes.