Step-by-step explanation:
Volume of cylindrical container

Answer: He spends more time trading stickers by 15 minutes.
Step-by-step: 1 hour is 60 minutes. So 2 hours is 120 minutes. 120 minutes plus his 20 minutes is 140 minutes. Subtract it from his homework time and he spends 15 more minutes trading stickers.
Because if it was even, then it would be divisble by 2 which makes it not prime