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?
Answer & explanation
Correct answer: option 4
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