Here's another way to look at the problem if you didn't know the formulas.
If the sequence started at 2, 2,4,6,8,...246, you would have 246/2=123 terms.
The sequence started at 6, so we have 123-2=121 terms.
The sequence is 6,8,10,12,...,240,242,244,246.
There are 121 terms. It is easy to find the 61st term: add and subtract the same number from 12 and 240 respectively until you find the middle term.
12+114=126 and 240-114=126, so 126 is the middle term.
Add the first and the last terms, add the second and the next to the last terms, and so on.
6+246=252, 8+244=252; you will always get 252.
There are 60 pairs that add up to 252(120/2=60 plus the middle term of 126)
60*252=15,120, 15,120+126=15,246 feet for the sum.
I would like to add something to my solution that came to me an hour ago. The average of all the numbers is 126; the average of all the pairs(252) is 126; the middle number therefore has to be 126-no work involved in determining the middle number !
Also, there are 60 groups of 252 or 120 groups of 126 plus the middle number which is 126 so we have 121 groups of 126:121*126=15,246 !