Multiple choice

In a pair of 6 numbers, the average of the first four numbers is 10 and the average of the last four numbers is 8. If the average of all 6 numbers is 9, find the average of the third and fourth numbers.

  1. 7

  2. 8

  3. 9

  4. 10

  5. 11
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C Correct answer
Explanation

Let 6 numbers be a,b,c,d,e,f. Given: (a+b+c+d)/4=10 so a+b+c+d=40. (c+d+e+f)/4=8 so c+d+e+f=32. Total average 9 means sum=54. Adding first two equations: a+b+2c+d+e+f=72. Subtracting total: (a+b+2c+d+e+f)-(a+b+c+d+e+f)=72-54, giving c+d=18. Average of c,d = 18/2=9.

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