In a survey of a city, it was found that 90 percent of the people in the city own a refrigerator and 15 percent own a washing machine. If everybody owns at least one appliance, what percentage owns both ?

$$\eqalign{ & n\left( A \right) = 90 \cr & n\left( B \right) = 15 \cr & n\left( {A \cup B} \right) = 100 \cr & {\text{So, }} \cr & {\text{n}}\left( {A \cap B} \right) \cr & = n\left( A \right) + n\left( B \right) - n\left( {A \cup B} \right) \cr & = 90 + 15 - 100 \cr & = 5 \cr} $$
∴ Percentage of people who own both = 5%