The average of thirteen numbers is 9. The average of first six numbers is 5 and the average of last six numbers is 7. What is the middle number?  Pick one:
A) 47
B) 45
C) 25
D) 37
E) 44
The total of thirteen numbers = 13X9 = 117   The total of first 6 and last 6 numbers is = 6 X 5+6 X 7 = 72   So, the middle number is  (117 - 72 )= 
<gadget id="calculator"> (117 - 72 )</gadget>
<output>45</output>
45   45
<result>B</result>
The average of thirteen numbers is 9. The average of first six numbers is 5 and the average of last six numbers is 7. What is the middle number?