Let the number of girls be g and the number of boys be b.

Overall average marks in English:

$9g+8b=8.8$ Simplifying this:

$9g+8b=8.8g+8.8b$

$0.2g=0.8b$ (Equation 1) $g=4b$(Equation 1) Overall average marks in Hindi: $g+b/8g+7b =x$ Substitute the value of g from Equation 1:

$4b+b/8(4b)+7b =x$

$x=7.8$

Method II:

Using the allegation method for the average marks in English:

Average marks of girls = 9 Average marks of boys = 8 Overall average = 8.8 The difference between the overall average and the girls' average is $9-8.8=0.2$, and the difference between the boys' average and the overall average is $8.8-8=0.8$.

Thus, the ratio of girls to boys is $0.8:0.2=4:1$.

Now, to find the average marks in Hindi, we calculate: $x= 4+1/(4\times 8)+(1\times 7)$ ​

Hence, the value of x is 7.8.