The average income of 40 persons is 4200 Rs. and that of an other 35 persons is 4000 Rs. The average income of the whole group is :
[A] $4108 \frac{1}{3}Rs.$
[B] $4106 \frac{2}{3}Rs.$
[C] $4100 Rs.$
[D] $4106 \frac{1}{3}Rs.$