There are seven numbers. The average of the first four numbers is 8.5, and the average of the last four numbers is 10. The average of these seven numbers is 6.9. What is the fourth number?

There are seven numbers. The average of the first four numbers is 8.5, and the average of the last four numbers is 10. The average of these seven numbers is 6.9. What is the fourth number?

9, then the sum of the seven numbers is 6. 9 × 7 = 48. 3
The average of the first four numbers is 29, and the sum of the first four numbers is 8.5 × 4 = 34
The average of the last four numbers is 10, and the sum of the last four numbers is 10 × 4 = 40
The sum of the first four numbers + the sum of the last four numbers = 74 = the sum of seven numbers + the fourth number
So the fourth number is 74-48.3 = 25.7