In a group of 70 people, 37 like coffee, 52 like tea and each person likes at least one of the two drinks. How many people like both coffee and tea?

Solution: Number of person who like coffee, $n\left(A\right)=37$
Number of person who like tea, $n\left(B\right)=52$
As each person likes at least one of the two drinks,
$n\left(A\cup B\right)=70$Number of person who like both coffee and tea,
$n\left(A\cap B\right)=n\left(A\right)+n\left(B\right)-n\left(A\cup B\right)=37+52-70=19$