A group of friends guessed the age of their friend, Sam. The guesses were 35, 40, 45, 48, 50, 51, 52, 54, 55, 60, 65, and 70. Sam says, "At least half of you guessed too low, exactly two of you are off by one, and my age is a prime number." How old is Sam?
51
53
55
57
47
