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 ?  Pick one.
A) 30
B) 18
C) 10
D) 9
E) 5

Rationale: "( average production for n days ) * n = ( total production for n days ) - - > 60 n = ( total production for n days ) ; ( total production for n days ) + 90 = ( average production for n + 1 days ) * ( n + 1 ) - - > 60 n + 90 = 65 * ( n + 1 ) - - > n = 5 . or as 30 extra units increased the average for n + 1 days by 5 units per day then 30 / ( n + 1 ) = 5 - - > n = 5 . answer : e ."
Answer: E