The average of 13 numbers is 17.8. If the average of the first seven numbers is 17.6 and that of the last seven numbers is 17.4,then the middle number is?  Choose the correct choice from the following choices.
A) 11.8 B) 11.9 C) 12.2 D) 13.6 E) 12.7
Middle number = 17.6*7 + 17.4*7- 17.8*13 = 123.2+121.8-231.4= 
<gadget id="calculator">123.2+121.8-231.4</gadget>
<output>13.6</output>
13.6   Answer is 13.6
<result>D</result>
The average of 13 numbers is 17.8. If the average of the first seven numbers is 17.6 and that of the last seven numbers is 17.4,then the middle number is?