In the above video on what basis we had taken " A " = 10
In these type of questions, we have to find the deviations di by using di = xi – A, i = 1, 2, 3, ..., n
and then the sum of these deviation i.e. 
.
We try to make this sum as small as we please for simple calculations, that's why we assume
the assumed mean in such a way so that few values of di are positive and the others are
negative.
The assumption of assumed mean depends on your choice. You can choose this value 12 or 15
or any other value as per your choice. Its not necessary to take assumed mean = 10 as taken in
the video. Answer will be same in all cases.