A question in a new physics textbook we are using in school tasks readers with determining the path difference and them commenting on whether it would result in constructive or destructive interference. The path difference in this question comes out to be 0.4m. The textbook says that this results in constructive interference. How do we decide this when the path difference is not, 0, 0.5, 1, 1.5, etc? The soundwaves have a wavelength of 0.4m.