For the past n days, the average (arithmetic mean) daily production at a company was 70 units. If today's production of 90 units raises the average to 75 units per day, what is the value of n ?
Select the correct option.
A) 30	B) 18	C) 10	D) 9	E) 3

Rationale: "( average production for n days ) * n = ( total production for n days ) - - > 70 n = ( total production for n days ) ; ( total production for n days ) + 90 = ( average production for n + 1 days ) * ( n + 1 ) - - > 70 n + 90 = 75 * ( n + 1 ) - - > n = 3 . answer : e ."
Answer: E