Before it charged, the battery was at 23%. In half and hour, it went to 89%. So, it rose 89-23, or 66% in half an hour. Now that we know how much it rose and that there are 30 mins in half an hour, we have to divide 66% by 30 to figure out the answer.
66/30 is 2.2.
So, the battery rose at 2.2% per minute.
Hope this helps!