The average of one group of numbers is 4.
A second group contains twice as many numbers and has an average of 10.
The average of both groups combined is:
. . (a) 5. . (b) 6. . (c) 7. . (d) 8. . (e) 9
The first group has n numbers and a total of T1.
. . Hence: .(T1)/n .= .4 . → . T1 = 4n .
The second group has 2n numbers and a total of T2.
. . Hence: .(T2)/2n) .= .10 . → . T2 .= .20n .
Add  and : .T1 + T2 .= .4n + 20n .= .24n
The total of both groups is: 24n
There are: .n + 2n .= .3n numbers in the two groups.
Therefore, the average of both groups is: .24n/3n .= .8 . . . answer choice (d)