If a = (1/5)b and c = 9a, then which of the following represents the average (arithmetic mean) of a, b, and c, in terms of a ?	Choices
A) a + 4  B) (11/3)a  C) 4a  D) 5a  E) (7 1/4)a
Official Answer: D   The average of the three variables isa + b + c/3. However, we need to solve in terms of a, which means we must convert b and c into something in terms of a.   Were told that a =1/5b, which is equivalent to b = 5a. We can plug that in and simplify the average to:a + 5a + c/3   We also know that c = 9a, which we can plug directly into the average expression:   a + 5a + 9a/3   = 15a/3 = 5a, choice5a.15 /3= 
<gadget id="calculator">15 /3</gadget>
<output>5</output>
5a, choice5a.
<result>D</result>
If a = (1/5)b and c = 9a, then which of the following represents the average (arithmetic mean) of a, b, and c, in terms of a ?