Which of these numbers equals the sum of all its factors apart from itself?

List the factors below each number and add them. For 28: 1 + 2 + 4 + 7 + 14 = 28, so it matches exactly. The others overshoot: 24 gives 1 + 2 + 3 + 4 + 6 + 8 + 12 = 36, 30 gives 1 + 2 + 3 + 5 + 6 + 10 + 15 = 42 and 36 gives 1 + 2 + 3 + 4 + 6 + 9 + 12 + 18 = 55. Including the number itself in the sum would make every total too big, so it must be left out.