There is a river of 100 m wide. Two persons A and B are located at same position. Person A can observe frequency greater than 200 Hz and person B less than 200 Hz. A sound source of frequency 200 Hz, which is located at point S, perpendicularly with the velocity of 3m/sec, where as velocity of river is 4m/sec, towards right. For what duration person A observes the signal?