Answer = 67.5
To solve this you put the number in order so (17 42 53 82 97 102) then you count them off by removing them left to right, first 17, then 102, then 42, then 97, then you are left with 2 numbers in the middle so you average them by adding them together and then dividing them by 2 to get 67.5. If you are left with one in the middle then that is your answer.
