Arithmetic

Let m be the mean of the three numbers. Then the least of the numbers is m-10 and the greatest is m + 15.  The middle of the three numbers is the median, 5. So ,  1/3 [ (m-10) + 5 + (m+15)] = m, which implies that m = 10 . Hence, the sum of the three numbers is 3 × 10 = 30.