In a class of 40 students, ratio of boys and girls is 3:2 and the average marks scored by boys is 42 and that by girls is 46. Then the average marks scored by the whole class is :

Let the number of boys be $B$ and the number of girls be $G$.

From the ratio, we have $B:G = 3:2$.

Let $B = 3x$ and $G = 2x$.

Since the total number of students is 40, we have:

$3x + 2x = 40$

$5x = 40$

$x = 8$.

Thus, $B = 3 \times 8 = 24$ and $G = 2 \times 8 = 16$.

The total marks scored by boys is $24 \times 42 = 1008$ and by girls is $16 \times 46 = 736$.

The total marks for the class is $1008 + 736 = 1744$.

The average marks for the whole class is:

$\frac{1744}{40} = 43.6$.

Therefore, the average marks scored by the whole class is $43.6$.