After the overnight trip to the beach, what does Gene decide Finny is trying to do to him? What leads Gene to this conclusion?

Gene wakes up at sunrise on the beach; he watches dawn break for the first time, while Finny is still sleeping. Gene realizes that he has a math exam in three hours, exactly the amount of time it will take to get back to Devon from the beach; he makes it back in time, but fails the test‹it is, according to him, the first test he has ever failed.

Gene, an academic perfectionist, laments his poor performance on the test to Finny; Finny mocks Gene's ambition to be first in their class, and Gene begins to believe that Finny doesn't want him to do well in school, so that he will come out ahead. Finny excels in athletics, and is definitely the best in the school; Gene knows that he can be the best in the school in academics, but thinks that Finny's high-jinks and his attempts to take up Gene's time are Finny's attempts to make sure that he comes out ahead in the relationship. Gene's jealousy, whether merited or not, begins to take him over; he decides that he cannot trust his "friend," and Finny's statement, that had been so touching the previous evening, of Gene being his best friend, Gene now believes to be false.