Jamal has a very flexible summer job.  He works every day but is allowed to take a day off anytime he wants.  His friend Don suggests they take off work on Tuesday to go to the amusement park.  The admission charge for the amusement park is $15 per person, and it will cost them $5 each for gas and parking.  Jamal loves amusement parks and a day at the park is worth $40 to him.  However, Jamal also enjoys his job so much that he would actually be willing to pay $5 per day maximum to do it. If Jamal earns $10 if he works, what is the cost and benefit of going to the park? Should he go to the amusement park?

不知道benefit怎么算，要不要把worth减去机会成本