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

Rationale: "( average production for n days ) * n = ( total production for n days ) - - > 40 n = ( total production for n days ) ; ( total production for n days ) + 90 = ( average production for n + 1 days ) * ( n + 1 ) - - > 40 n + 90 = 45 * ( n + 1 ) - - > n = 9 answer : d ."
Answer: D