Grandalf ambushedDiscrete Mathematics Level 1
Grandalf the Grand Wizard was ambushed by an army of trolls. He cast a series of three fireballs in different directions. The fireball to the north killed 20 trolls, while the fireball to the east killed 13 trolls and the fireball to the south killed 15 trolls. How many trolls did Grandalf manage to kill with the three fireballs?