please solve this.

Qs 3. In Singapore you can ride the very efficient Metro trains with only a prepaid card of $ 25 which just keeps reducing the balance as you ride the trains. You can have the card refilled (the agent puts the card in a machine and the new total amount shows) but only in $10 increments or of course, you can buy a new one for $25. Your MA professor has $17 left on his card (enough for his remaining time in Singapore) when he has a sudden urge for Singapore's famous mouth watering chilli crab but he realizes he has no cash on him and the crabwaalah will only accept cash. A passerby who has just seen the value on the professor's card as he left the station (you must put the card in a card reader to exit and the value remaining is displayed) offers to buy his card for $12 which will buy a small crab a) What is the opportunity cost of not selling the card? (Answer may be qualitative) b) If the professor will need to buy another card to carry on using the trains while he is in Singapore what is the Relevant Cost of the crab? (16 pts)