If x is the average (arithmetic mean) of m and 9, y is the average of 2m and 15, and z is the average of 3m and 18, what is the average of x, y, and z in terms of m? a) m+6 b) m+7 c) 2m+14 d) 3m+21

For this case we have to:
 x = (m + 9) / 2
 y = (2m + 15) / 2
 z = (3m + 18) / 2
 The average of x, y, and z is:
 Average = (x + y + z) / 3
 Average = (((m + 9) / 2) + ((2m + 15) / 2) + ((3m + 18) / 2)) / 3
 Average = (((m + 9)) + ((2m + 15)) + ((3m + 18))) / 6
 Average = (6m + 42) / 6
 Average = (m + 7)
 
 The average of x, y, and z in terms of m is
 B) m+7

That they are numbers and related to each other.