A soldier walks towards a high wall taking 120 steps per minute.When he is at a distance of 90 m from the wall he observes that echo of step coincides with next step.The speed of saound must be:
a)340 m/s
b)330 m/s
c)300 m/s
d)360 m/s

number of steps = 120
time taken = 60 seconds
therefore , number of steps in one second = 120/60 = 2 steps

distance travelled by the sound wave  = 2 x 90
                                                          = 180 m
In one seconds 2 steps are heard
Therefore, 1step is heard in 1/2 = 0.5 seconds

speed = distance / time
          = 180 / 0.5
          = 360 m/s
therefore option d is correct