Let's use a couple of examples and see if we can generate an answer
Let one series be
1, 3, 5 , 7 , 9
The mean is [ 1 + 3 + 5 + 7 + 9 ] / 5 = 25 / 5 = 5 = y
Sum of largest and smallest values = 10 = 2y
Let another series be
11, 13, 15, 17, 19
The mean is 15
So....it appears that the mean = the median value
Sum of largest and smallest value = 30 = 2y
So...... it appears that the sum of the largest and smallest value = 2 * average = 2y