$\Rightarrow \mathrm{d}=\mathrm{v}\left(\mathrm{t}_{0}+\mathrm{t}\right)$
$\Rightarrow(v-u) t_{0}+v t=0$
$t_{0}=\frac{v t}{u-v}$
$d=\frac{u v t}{u-v}$

$y = a\sin (kx + \omega t)$ ......$(1)$
$y = a\sin (\omega t - kx)$ ......$(2)$
$y = a\cos (kx + \omega t)$ ......$(3)$
$y = a\cos (\omega t - kx)$ ......$(4)$
emitted by four different sources ${S_1},\,{S_2},\,{S_3}$ and ${S_4}$ respectively, interference phenomena would be observed in space under appropriate conditions when
