.
In both posts by @ewatrrr and @MathLover1, everything including the problem setup, the solution and the answer,
is WRONG, INCORRECT and IRRELEVANT,
because these ladies misread the problem and did not take a labor to read the problem attentively and to understand its meaning.
I came to bring the correct solution.
Let x be the number of girls.
Then the total students is (1200 + x).
The total of those who pays their school fees is (0.5*1200 + 0.4x) = (600 + 0.4x).
The equation is
= 0.46.
To solve it, multiply both sides by the denominator and simplify
600 + 0.4x = 0.46*(1200 + x)
x = = 800.
ANSWER. The number of girls in the school is 800.
Solved.