Here, given distance, d = 45 km = 45000m and,speed of signal WAVE, V = 3 X 10^8 now, total distance covered by signal = 2d (: - the signal reaches plane and returns) now, we KNOW v = d/ t t = d/ v t = 2 x45000/3 x10^8 = 3 x 10^-4 HOPE THIS HELPS YOU ;-)