Explanation: I am sure you missed an extra 10 in your problem. There are supposed to be four 10's (Well at least in my problem). The rest you know, the answer are stated for the set of data: 10, 1, 10, 15, 1, 7, 10, 10, 1, 6, 13.
Well the first thing that came to my mind was to do rise/run and doing that I got 6/5 which I added together to get 11 but since that's obviously not correct I assumed it was a little under so the answer I got was 7.8