Prove that the problem of determining the average completion time has the greedy-choice property

Consider the problem of scheduling tasks on a single processor, without swapping, in order to minimize the average completion time of all the tasks. For example, if tasks a_1 and a_2 take t_1 = 2 seconds and t_2 = 3 seconds to complete, then the average completion time of performing these two tasks is minimized when a_1 is performed first: a_1 would finish at time 2, and a_2 would finish at time 5, for an average completion time of (2 + 5)/2 = 3.5. If a_2 were performed first, the average completion time would be (3 -+- 5)/2 = 4. Given tasks a_1, a_2, ..., a_n with running times t_1, t_2, ... t_n, describe an algorithm that will determine the ordering of these tasks that minimizes the average completion time. Give an asymptotic estimate for the running time of your algorithm as a function of n