For the past n days, the average (arithmetic mean) daily production at a company was 50 units. If today's production of 90 units raises the average to 58 units per day, what is the value of n ? Choices:
A) 30
B) 18
C) 10
D) 9
E) 4
(average production for n days) * n = (total production for n days) --&gt; 50n=(total production for n days);   (total production for n days) + 90 = (average production for n+1 days) * (n+1) --&gt; 50n + 90 = 58 * (n+1) --&gt; n=4.   Or as 40 extra units increased the average for n+1 days by 8 units per day then 40/(n+1)=8 --&gt; n=4.   Answer: 4.
<result>E</result>
For the past n days, the average (arithmetic mean) daily production at a company was 50 units. If today's production of 90 units raises the average to 58 units per day, what is the value of n ?