A set of consecutive positive integers beginning with 1 is written on the blackboard. A student came along and erased one number. The average of the remaining numbers is . What was the number erased?
After one value is removed: Since all of the values are integers, the sum here must be an integer. sum=(number)×(average) Since the average , and the sum be an integer, the number of integers must be a multiple of 17, For any evenly spaced set, average = median. After one of the consecutive integers is removed , most of the remaining set will be evenly spaced. As a result , the average of the remaining set will still be close to the median. Implication: The number of integers , with the result that will be close to the median of the 68 mostly consecutive integers. ∴ Sum Original set: Since 68 integers remain after one of the integers is removed, the original set contains 69 integers. Sum of the first n positive integers ∴ Sum Removed integer = original sum - sum after one integer is removed .