Final answer:
The propagation speed of a 5 MHz wave remains unchanged at 1.8 km/s when the duty factor changes to 0.5, since the duty factor does not affect the wave's speed through the medium.
Step-by-step explanation:
The propagation speed of a wave refers to how fast a wave travels through a medium and is determined by the properties of the medium, not by the wave's frequency or duty cycle. The duty factor (or duty cycle) of a wave refers to the fraction of one period in which a signal or system is active. A duty factor of 0.5 means that the wave is active 50% of the time and inactive the other 50%. This does not affect the speed at which individual waves travel through the medium. Therefore, if the speed of a 5 MHz continuous wave is 1.8 km/s, changing the duty factor to 0.5 will not change the propagation speed. The new propagation speed remains the same, at 1.8 km/s.