The average of 18 numbers is 42.  The average of the last 10 numbers 40 and that of the first five numbers is 44, then the seventh number is 6 less than the sixth and 7 less than the 8th number.  Then the average of 6th and 8th number is?

47.5

⇒ 10 × (-42 + 40) + 5 × (44 - 42)

⇒ 10 × -2 + 10 = -10

Deviation is = -10

Now, sum of three numbers = 42 × 3 + 10 = 136

6th    7th      8th

 x      x-6      x+1

3x - 5 = 136

x = 47

So, 6th = 47

8th = 48

Average = \(\frac{47 + 48}{2}\) = 47.5