Mathematics for IGCSE & O level - Sets (Section 2 - No. 9)
Which of the following sets is empty?
The set of all positive integers.
The set of all days of the week.
The set of all triangles with four sides.
The set of all prime numbers between 10 and 20
Explication
A set containing all triangles with four sides has no elements as a triangle can only have 3 sides