For the past n days, the average (arithmetic mean) daily production at a company was 40 units. If today's production of 90 units raises the average to 45 units per day, what is the value of n ?  Options:
A) 30 B) 18 C) 10 D) 9 E) 7
(average production for n days) * n = (total production for n days) --&gt; 40n=(total production for n days);   (total production for n days) + 90 = (average production for n+1 days) * (n+1) --&gt; 40n + 90 = 45 * (n+1) --&gt; n=9   Answer: 9.
<result>D</result>
For the past n days, the average (arithmetic mean) daily production at a company was 40 units. If today's production of 90 units raises the average to 45 units per day, what is the value of n ?