The average of twelve numbers is 70. The average of the first four numbers is 75 and that of the next five numbers is 68. The 10th number is three times the 12th number and the 11th number is 5 less than the 12th number. What is the average of the 11th and 12th number?

38.5

Total =  12 × 70 = 840

First 4 =   4 × 75 = 300     

Next 5 = 5 × 68 = 340   

⇒ 840 - 300 - 340 = 200

10th :   11th    :   12th                                    

3x         x - 5         x       

By adding all ,

5x - 5 = 200                                       

5x = 205    

x = 41

11th number = 36   

12th no. = 41 

⇒ Avg. of 11th and 12th = \(\frac{36 + 41}{2}\) = 38.5