Given that a is the average (arithmetic mean) of the first six positive multiples of six and b is the median of the first twelve positive multiples of six, what is the ratio of a to b?  Pick
A) 3:4  B) 10:13  C) 5:6  D) 13:10  E) 7:13
The first nine positive multiples of six are {6, 12, 18, 24,30, 36}   The first twelve positive multiples of six are {6, 12, 18, 24, 30,36,42, 48, 54, 60, 66, 72}   Both sets are evenly spaced, thus their median=mean:   a=21 and b=(36+42)/2= 
<gadget id="calculator">(36+42)/2</gadget>
<output>39</output>
39 --&gt; a/b=21/39=7/13.   Answer: 7:13.
<result>E</result>
Given that a is the average (arithmetic mean) of the first six positive multiples of six and b is the median of the first twelve positive multiples of six, what is the ratio of a to b?