Let d be the common difference and a be the first term of AP The fifth term of the series will be a + 4d. If a + 4d is divisible by d then a should also be divisible by d. Hence the cases are: d = 1, a = 1,2, 3......16 d =2, a = 2, 4, 6......12 d = 3. a = 3, 6 d = 4, a = 4 So the answer is 16 + 6 + 2 + 1 =25.