Consider the following data:

 

What is the value of x in the above table?

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

So by the condition given in question we have 9g + 8b = 8.8 (b + g). That gives us ratio b:g = 1:4. It means that if there are p boys number girls will be 4p, where p is any positive integer.

Again by the condition given in question 8×4p + 7×p = X (4p + p)

=> 32p + 7p = 5p × X => 39p = 5pX => 39 = 5X or X = 39/5 or X = 7.8.