a splash is heard after 4.22sec. after the stone is dropped into a well of depth 78.4m.

Let the velocity of sound be v. Time taken by stone to reach bottom of well be t.

We have:

-78.4=0t-1/2*9.8*t²

t=4 s

 

Time taken by sound to reach top,t'=4.22-4=0.22 s

Hence,

Speed of sound,v=78.4/0.2556.36.2=356.36 ms^-1