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