I think that x would equal 1.5 :)
Let
be the 20 marks of the boys, and
be the 10 marks of the girls.
We know that the global mean was 70, meaning that

Multiplying both sides by 30 we deduce that the sum of the scores of the whole classroom is

By the same logic, we work with the marks of the boys alone: we know the average:

And we deduce the sum of the marks for the boys:

Which implies that the sum of the marks of the girls is 
And finally, the mean for the girls alone is

Answer:
I am still trying to pick between third-person limited and third-person omniscient...Let me know which one is right :)
Step-by-step explanation:
Im positive its false because they dont add up
The first step would be to add 7 to both sides.
3n-7=30
add 7
3n=37
divide by 3
n=12.33