Answer:
350 m/s
Explanation:
In order for Boy to hear the sound, the sound has to travel from his hands to the cliff, and then BACK to him from the cliff to his ears.
So the sound has to travel (2 x 420m) = 840 m.
Speed = (distance) / (time)
Speed = (840 m) / (2.4 sec)
Speed = 350 m/s
