Answer:
12 hours
Explanation:
Every 2 hours the temp. will rise 5°F, thus that means in 6 hours the room will rise exactly 30°F. Since we muliplied 2.5 by 2 to get 5°F per 2 hours, we have to multiply 6 by 2, to get 12 hours in total. Or you can just do repeated addition until you get to 84°F.