2 college students accidentally miss the math final exam

The next day they both went to plead with their professor. He was feeling pretty good that day so he allowed them to retake it. He told them to both come back tomorrow for an oral exam. When they both showed up he told one of them to wait outside while he tests the other. So one enters and the other puts his ear to the door to listen. The professor begins asking the question:

"You are riding in a train cart and you get too hot. What do you do?"

The student replies," I open the window."

"Ok. Now that window is 2 feet wide and 3 feet high. The train is traveling 50 mph going north and the wind is blowing at 15 mph due east. How long will it take for new air to replace the old air in the cart?"

The student is clearly confused at this impossible question and just answers,"I don't know." So the professor gives him an F, dismisses him, and calls in his friend.

He begins asking his friend "you are riding in a train cart and it gets too hot. What do you do?

He says,"I will take my jacket off."

"Ok. But its still too hot. What do you do?"

"I take my shirt off."

"I understand but its very, very hot."

"I will just get naked."

"Ok. But there's a guy in front of you getting a hard on by watching you strip naked!"

The student replies, "Professor, the entire train can fuck me in the ass I am NOT opening that window!"