The average (arithmetic mean) of four numbers is 4x + 9. If one of the numbers is x, what is the average of the other three numbers? Select.
A) x + 1
B) 3x + 3
C) 5x + 1
D) 5x + 4
E) 5x + 12
The sum of all four numbers is 4 * (4x + 9) = 16x + 36.   If one of the numbers is x, the sum of the other 3 numbers is 15x+36.   The average of the remaining three numbers is (15x+36)/3 = 5x+12.   The answer is 5x + 12.
<result>E</result>
The average (arithmetic mean) of four numbers is 4x + 9. If one of the numbers is x, what is the average of the other three numbers?