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 54 units per day, what is the value of n ?
Choose the correct choice from the following choices:  A) 30  B) 18  C) 10  D) 9  E) 7
(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 = 54 * (n+1) --&gt; n=9.   Or as 40 extra units increased the average for n+1 days by 4 units per day then 40/(n+1)=4 --&gt; n=9.   Answer 9.
<result>D</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 54 units per day, what is the value of n ?