There are 30 students in a class. 15 study woodwork and 13 study metalwork. 6 study neither of the two subjects. How many students study woodwork but not metalwork?
Worked solution (try it first)
30−6=24 students study at least one of the two subjects.
Add the two subject totals and take away 24 to find how many study both: 15+13−24=4.