Well, if you are to find the arithmetic mean (average) of that set of values, then add them up, and divide the resulting sum by the number of elements.

I can't get the mean of this set
[1,2,3,8,7}
Thanks in advance
I got 21 ,when I added it up and there is 5 elements in the set .
Should you just right 4.2 or is that wrong?