40
There is an interval of 3 between the numbers 1+3=4 4+3=7 7+3=10 10+3=13 13+3=16 16+3=19 19+3=22 22+3=25
7
31
They are: 7, 10, 13, 16, and 19
The following is the answer.
{1}, or {1, 13} or {4, 16, 22} are three possibilities
40
10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20
There is an interval of 3 between the numbers 1+3=4 4+3=7 7+3=10 10+3=13 13+3=16 16+3=19 19+3=22 22+3=25
2n+1
7
31
22
The median of {10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20] is the 6th values which is 15.
They are: 7, 10, 13, 16, and 19
7