The average of 30 numbers is 40 and that of other 40 numbers is 30. The average of all the numbers is :
[A]$34$
[B]$34\frac{2}{7}$
[C]$34.5$
[D]$35$

$\mathbf{34\frac{2}{7}}$
Average of all numbers =
$= \frac{30\times 40 + 40\times 30}{70}$
$= \frac{2400}{70} = 34\frac{2}{7}$
Hence option [B] is correct answer.