One student proposed some doubts about one of the exercises:

If Peter goes to watch a game, it costs 5 to buy the ticket (including 1 hour queuing), and the game lasts 2 hours. Peter could do a part-time job which gives 10 per hour. The price of the ticket is 30 for outsiders, and one outsider would like to pay 20 to buy from Peter. What is the opportunity cost if Peter watches the game?

A. 35

B. 40

The answer is 35. But students thought it shall be 40. I forgot where did I find the exercise. Now I am also confused. Any comments?