Q32 Darmok and Jalad's arrival times X and Y have the joint density function if 0

Darmok and Jalad's arrival times X and Y have the joint density function if O S x q and O S y S 4 O otherwise Darmok is trying to break his habit of arriving late. He has agreed to donate 120 credits to a local charity for each hour Jalad has to wait for him (prorated across partial hours). Assuming that this incentive has no effect on their arrival times, what is the expected donation?