The average of seven numbers is 18. The average of first three numbers is 16 and the average of last three numbers is 20. What is the middle number? Choose one.
A) 20
B) 22
C) 25
D) 18
E) 24
The total of seven numbers = 7X18 = 126   The total of first 3 and last 3 numbers is = 3 X 16+3 X 20 = 108   So, the middle number is  (126 - 108 )= 
<gadget id="calculator"> (126 - 108 )</gadget>
<output>18</output>
18   18
<result>D</result>
The average of seven numbers is 18. The average of first three numbers is 16 and the average of last three numbers is 20. What is the middle number?