Final answer:
The speed of the jet is 100 km/hr.
Step-by-step explanation:
To find the speed of the jet, we can use the principle of conservation of momentum.
The momentum of the jet and the hot air must be equal and opposite in direction.
Given that the speed of the jet is 100 km/hr and the speed of the hot air is 80 km/hr, we can set up the equation:
100 km/hr * (mass of jet) = -80 km/hr * (mass of hot air).
Since the masses are the same for both the jet and the hot air, we can simplify the equation to:
100 km/hr = -80 km/hr.
Hence, the speed of the jet is 100 km/hr.