A sound from a single source can reach point O by two different paths. One path is 20 m long and the other is 21 m long. The two waves interfere destructively at the point O. Assume there are no phase changes due to reflections, what is the minimum possible frequency of the source? Take the speed of sound to be 340 m/s

A sound from a single source can reach point O by two different paths. One path is 20 m long and the other is 21 m long. The two waves interfere destructively at the point O. Assume there are no phase changes due to reflections, what is the minimum possible frequency of the source? Take the speed of sound to be 340 m/s