So you know the speed of a wave, and speed is how much the wave moves per second. This wave travels 3x10^8 meters per second. So to find how for you traveled you need to know how many seconds the wave traveled for, 20 minutes is equal to 1200 seconds (times 20 minutes by 60). Then times the seconds by the speed (3x10^8 m/s)x(1200 s) = 3.6x10^11 meters
