College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 17

Waves from a radio station have a wavelength of 300 m. They travel by two paths to a home receiver 20.0 km from the transmitter. One path is a direct path, and the second is by reflection from a mountain directly behind the home receiver. What is the minimum distance from the mountain to the receiver that produces destructive interference at the receiver (Assume that no phase change occurs on reflection from the mountain.)