Semi-automatic weapons chamber a new round each time you fire. John loads his empty semi-automatic weapon with a 30-round magazine. He fires 18 rounds. He then reloads with a fresh 30-round magazine. He fires 10 rounds more than he did the first time. He then replaces the magazine with a fresh 30-round magazine and fires five more rounds. How many more rounds can he fire before he must reload?
a. 7 rounds
b. 12 rounds
c. 17 rounds
d. 22 rounds