Answer:
Water normally freezes at 0°C (32°F). Salt lowers the freezing temperature. (That is, it can remain a liquid at much lower temperatures.)
When sprinkled on ice, the salt lowers the freezing temperature of the water which effectively melts the ice when the salt dissolves into it. There is a limit to how low it can reduce the temperature, though. If the temperature drops below -9°C (15°F), it's too cold for the salt to dissolve into the ice.
When making ice cream, the salt lowers the temperature of the ice and water sufficiently enough to freeze the cream.
Inertia I think because I've heard it around school and in science
Average speed=total distance traveled/time. Total distance=the sum of the two short distances=79*2+77*3=389 km. Total time used to travel is 2+3=5 hr. Average speed is 389/5=77.8 km/hr.