Serena should buy the ticket for $70.
Step-by-step explanation:
Serena's reservation price is the maximum amount she is willing to pay for a ticket to see Hamilton, which is $75. However, when she tries to buy a ticket at the box office, they are sold out, which means that the only option left for her is to buy a ticket from a scalper. The scalper is offering to sell her a ticket for $70, which is less than her reservation price of $75. Therefore, buying the ticket for $70 is still worth it for Serena since it is less than her maximum willingness to pay.