For closed tube length available for resonance is
$l' = l \times \frac{{25}}{{100}} = \frac{l}{4}$
Fundamental frequency of water filled tube
$ \therefore$ $\,n = \frac{v}{{4l'}} = \frac{v}{{4 \times (l/4)}} = \frac{v}{l} = 2{n_0}$
==> $\frac{n}{{{n_0}}} = 2$
