Bullets fired, when can I hear it?

Classical Mechanics Level 3

The bullet of a poacher flying at the speed of $v=680\text{ m/s}$ passes the gamekeeper at a distance $d=1 \text{ m}$ . What was the distance (in meters) of the bullet from the gamekeeper when he began to sense its shrieking sound?

The speed of propagation of sound is $c=340 \text{ m/s}$ .

Bullets fired, when can I hear it?

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