Also number of loops = Number of antinodes.
Hence, with $5$ antinodes and hanging mass of $9 kg$.
We have $p = 5$ and $T = 9g$ ==> ${n_1} = \frac{5}{{2l}}\sqrt {\frac{{9g}}{m}} $
With $3$ antinodes and hanging mass $M$
We have $p = 3$ and $T = Mg$
==> ${n_2} = \frac{3}{{2l}}\sqrt {\frac{{Mg}}{m}} $
$ \because n_1 = n_2$
==> $\frac{5}{{2l}}\sqrt {\frac{{9g}}{m}} = \frac{3}{{2l}}\sqrt {\frac{{Mg}}{m}} $==> $M = 25\, kg.$