The speed of the hot air relative to the ground is 20 km/hr.
When a jet is cruising at a speed of 100 km/hr and ejects hot air in the opposite direction, the speed of the hot air is measured relative to the jet. If the speed of the hot air with respect to the jet is 80 km/hr, we can find the speed of the hot air relative to the ground by considering their combined velocities.
The speed of the hot air relative to the ground is obtained by subtracting the speed of the jet (which is in the opposite direction) from the speed of the hot air relative to the jet. Mathematically, it is expressed as:
Speed of hot air relative to ground=Speed of hot air relative to jet−Speed of jet
Substituting the given values:
Speed of hot air relative to ground=80km/hr−100km/hr=−20km/hr
The negative sign indicates that the hot air is moving in the opposite direction to the jet. Therefore, the speed of the hot air relative to the ground is 20 km/hr.