A source of sound S is travelling at 100/3 m/s along a road, towards a point A. When the source is 3 m away from A, a person standing at a point O on a road perpendicular to AS hears a sound of frequency v'. The distance of O from A at that time is 4 m. If the original frequency is 640 Hz, then the value of v' is (velocity of sound is 340 m/s)