The average of ten numbers is 72. The average of the first four numbers is 69 and that of the next three numbers is 74. The 8th number is 6 more than the 9th number and 12 more than the 10th number. What is the average of the 8th and 9th numbers?

77

Sum of all ten numbers= 72 x 10 = 720

Sum of first four numbers = 69 x 4 = 276

Sum of next three numbers = 74 x 3 = 222

Sum 8th, 9th & 10th number = 222

Lets assume 8th number is x then 9th & 10th numbers are x - 6 & x - 12

So, 3x - 18 = 222

x= 80

Accordingly, 8th & 9th numbers are 80 and 74

Avg. = 77