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 ?  Pick.
A) 30
B) 18
C) 10
D) 9
E) 7

Rationale: "( average production for n days ) * n = ( total production for n days ) - - > 50 n = ( total production for n days ) ; ( total production for n days ) + 90 = ( average production for n + 1 days ) * ( n + 1 ) - - > 50 n + 90 = 54 * ( n + 1 ) - - > n = 9 . or as 40 extra units increased the average for n + 1 days by 4 units per day then 40 / ( n + 1 ) = 4 - - > n = 9 . answer d ."
Answer: D