Two speakers are spaced 15 m apart and are both producing an identical sound wave. You are standing at a spot as pictured. What would be the frequency produced by the speakers to create perfectly constructive interference? Assume n = 1 and v = 343 m/s

The illuminance of a surface varies inversely with the square of its distance from the light source. if the illuminance of a surface is 120 lumens per square meter when its distance from a certain light source is 6 meters, by how many meters should the distance of the surface from the source be increased to reduce its illuminance to 30 lumens per square meter?