Elizabeth just cannot live the pirate life and keep that drop-dead gorgeous complexion witheut regular spa visits. She can go to the Montego Bay Retreat for a mud facial that lasts seven weeks, costing $91, or visit the crater of Mount Pelee for a lava exfoliation at $250. The exfoliation is good for 31 weeks. Elizabeth has a discount rate of 5% per week. Which spa is best for her?