To find the mean marks of the girls in the class, we can use the formula for the weighted mean:

Mean = (Sum of (Value * Weight)) / (Sum of Weights)

In this case, the value represents the marks and the weight represents the number of students.

Given that the mean of the marks of the entire class is 72, we can set up the equation:

72 = (Sum of (Value * Weight)) / (Sum of Weights)

Now, let’s consider the marks of the boys. The mean of their marks is 75, and there are 70 boys. So, we can add this information to the equation:

72 = (Sum of (Value * Weight)) / (Sum of Weights) 72 = (75 * 70 + Value * Weight of girls) / (70 + Weight of girls)

Simplifying the equation, we get:

72 = (5250 + Value * Weight of girls) / (70 + Weight of girls)

Now, we can solve for the mean marks of the girls by isolating the Value * Weight of girls term:

72 * (70 + Weight of girls) = 5250 + Value * Weight of girls

Expanding the equation:

5040 + 72 * Weight of girls = 5250 + Value * Weight of girls

Rearranging the equation:

Value * Weight of girls - Weight of girls = 5250 - 5040

Factoring out the Weight of girls:

Weight of girls * (Value - 1) = 210

Finally, we can solve for the mean marks of the girls by dividing both sides of the equation by (Value - 1):

Weight of girls = 210 / (Value - 1)

Since the number of girls is not given in the problem, we cannot determine the exact mean marks of the girls. However, we can see that the mean marks of the girls will be greater than 72, as the weight of girls is positive.