The average of 8 numbers is 20. The average of first two numbers is 151/2 and that of the next three is 211/3 . If the sixth number be less than the seventh and eighth numbers by 4 and 7 respectively, them the eighth number is :

Answer: Option C

Let the sixth number = x
Then the seventh = x + 4
And the eighth = x + 7
According to the question,
$$\Rightarrow 2\times \frac{31}{2}+3\times \frac{64}{3}+x+x+4+x+7$$        = 8 × 20
$$\begin{array}{rl}& \Rightarrow 31+64+3x+11=160\\ & \Rightarrow 106+3x=160\\ & \Rightarrow 3x=54\\ & \Rightarrow x=18\end{array}$$
∴ Eighth number x + 7 = 18 + 7 = 25