For the past n days, the average (arithmetic mean) daily production at a company was 60 units. If today's production of 90 units raises the average to 65 units per day, what is the value of n ? Choose the correct choice from the following answers:
A) 30  B) 18  C) 10  D) 9  E) 5
(average production for n days) * n = (total production for n days) --&gt; 60n=(total production for n days);   (total production for n days) + 90 = (average production for n+1 days) * (n+1) --&gt; 60n + 90 = 65 * (n+1) --&gt; n=5.   Or as 30 extra units increased the average for n+1 days by 5 units per day then 30/(n+1)=5 --&gt; n=5.   Answer: 5.
<result>E</result>
For the past n days, the average (arithmetic mean) daily production at a company was 60 units. If today's production of 90 units raises the average to 65 units per day, what is the value of n ?