Question

The sound of an explosion on the surface of lake is heard by a boatman 100m away and a driver 100m below the point of explosion.
(i) explain who will hear the sound of explosion first
(ii) if the sound takes t s to reach the man how much time will it take to reach the driver?
(speed of sound in air = 344 m/s
speed of sound in water = 1533 m/s )

Answer 1

For the boatman above the surface of water...

speed of sound in air = 344 m/s
distance covered = 100 m
time taken by sound = t sec
velocity = distance / time
 
 344 = 100 / t
  t  = 100 / 344
 t = 0.29 sec
 

Now, for the man below the surface of water

speed of sound in water = 1533 m/s
distance covered = 100 m
time taken = t

But velocity = distance / time
      1533 = 100 / t
         t   = 100 / 1533 
          t  = 0.06 sec

So , the man under the water hear the sound of the explosion first....

Answer 2

The word  is not  "driver", it is "diver" insider water/lake.

The time duration for sound of explosion to reach the boatman: 
              100 m / 344 m/s = 0.29 sec

The time duration for the sound of explosion to reach the diver traveling through water in the lake =  100 m / 1533 m/s = 0.065 sec

Since the speed of sound in water in the lake is more than the speed of sound in air, the sound will reach the diver first.