The waves from $S$ reach point $P$ directly following the path $SMP$ and being reflected from the ceiling at point $A$ following the path $SAP$.

$M$ is mid-point of $SP$ (i.e. $SM = MP)$ and $\angle \,SMA = {90^o}$

Path difference between waves $\Delta x = SAP - SMP$

We have $SAP = SA + AP = 2(SA)$

$ = $$2\sqrt {[{{(SM)}^2} + {{(MA)}^2}]}  =  2\sqrt {({{60}^2} + {{25}^2})}  =130m$

$\therefore $ Path difference $= SAP -SMP   = 130 - 120 = 10 m$

Path difference due to reflection from ceiling = $\frac{\lambda }{2}$

$\therefore $ Effective path difference $\Delta x = 10 + \frac{\lambda }{2}$

For constructive interference

$\Delta x = 10 + \frac{\lambda }{2} = n\lambda \Rightarrow (2n - 1)\frac{\lambda }{2} = 10(n = 1,\,\,2,\,\,3....)$

$\therefore $ Wavelength $\lambda = \frac{{2 \times 10}}{{(2n - 1)}} = \frac{{20}}{{2n - 1}}$.

The possible wavelength are ?$ = $$20,\,\,\frac{{20}}{3},\,\,\frac{{20}}{5}\,,\,\,\frac{{20}}{7}\,,\,\,\frac{{20}}{9}\,,$…..

$ = $ $20$$m$, $6.67$$m$, $4m,$$2.85\,m,$ $2.22$$m,$…..