Pens in Three Colours

Difficulty: Medium · Topics: Discrete Mathematics, Sets, Word Problems

Each of the 15 students in a class has blue, red or green pens in their pencil case. 2 students have pens of all three colours. 3 students have blue and red pens, 5 students have blue and green pens, and 4 students have red and green pens.

If 10 students have a blue pen and 7 students have a red pen, how many students have a green pen?