Correct Answer: 400 female

Explanation:

Let, in 1998 female students were x The male students were = (900 —x)

At 4% male students left, Among 100 male students left = 4

Among (900 ~x) male students left = $\frac{4⋅(900-x)}{100}$

= $\frac{900-x}{25}$

Again, 5% female students new,

Among 100 female students joined = 5

Among x female students joined = $\frac{5x}{100} = \frac{x}{20}$

According to the question,

$\frac{900-x}{25} = \frac{x}{20}$

=> 25x = 18000-20x

So, x = 400