The average of twelve numbers is 80 . The average of the first four numbers is 80 and that of the next five numbers is 74. The 10th number is two times the 12th number and the 11th number is 10 less than the 12th number. What is the average of the 11th and 12th number? 

65

Total =  12 × 80 = 960

Sum of First 4 numbers  =   4 × 80 = 320     

Sum of First 5 numbers  = 5 × 74 = 370   

Sum of remaining numbers are =  960 - 320 - 370 = 270

10th :   11th    :   12th                                    

2x         x - 10         x       

By adding all ,

4x - 10 = 270                                       

4x = 280    

x = 70

11th number = 60

12th no. = 70

⇒ Avg. of 11th and 12th = \(\frac{70 + 60}{2}\) = 65