Someone rides a motorcycle to work from place a to place B. He has to walk 40 kilometers of flat road, 48 kilometers of uphill road and 36 kilometers of downhill road. His speed on the flat road is per hour 20 km, 2 km faster than 1.2 times the uphill speed, and the downhill speed is 1.6 times the uphill speed. How many hours does it take for him to go back and forth? (can you explain each step clearly?)

Someone rides a motorcycle to work from place a to place B. He has to walk 40 kilometers of flat road, 48 kilometers of uphill road and 36 kilometers of downhill road. His speed on the flat road is per hour 20 km, 2 km faster than 1.2 times the uphill speed, and the downhill speed is 1.6 times the uphill speed. How many hours does it take for him to go back and forth? (can you explain each step clearly?)

40×2÷20+（48+36）÷【（20-2）÷1.2】+（48+36）÷【（20-2）÷1.2】÷1.6
=4+84÷15+84÷15÷1.6
=4+5.6+3.5
=13.1 hours

A car drives from a place to B 320km away at the speed of 80km per hour. (1) write the functional relationship between the distance S1 from a place and the time t, and draw the functional image. (2) write the functional relationship between the distance S2 from B place and the time t, and draw the functional image

(1) So when t = 4, S1 = 320, so the line passes through points (0, 0) and (1, 80). The image is as shown in the figure. (2) according to the meaning of the problem, S2 = 320-80t

When a motor vehicle is driving on a highway, the minimum distance from the vehicle in front of the same lane shall not be less than how many meters

Article 80 when a motor vehicle is driving on an expressway at a speed of more than 100 km / h, it shall keep a distance of more than 100 meters from the vehicle in front of the same lane. When the speed is less than 100 km / h, the distance between the motor vehicle and the vehicle in front of the same lane may be appropriately shortened, but the minimum distance shall not be less than 50 meters

For the sake of safety, the necessary distance should be kept between the vehicles driving on the expressway. The maximum speed limit of a certain expressway is known
For the sake of safety, the necessary distance should be kept between the vehicles driving on the expressway. It is known that the maximum speed limit of a certain expressway is v = 120km / h. suppose that the vehicle in front suddenly stops and the driver in the rear finds this situation, the time (reaction time) t = 0.5s from the time when the vehicle starts to decelerate (i.e. the acceleration of the vehicle when braking is a = 4m / s * 2, At least how far should the garage stay on the highway?

LS is a bit wrong - the speed of 0.5s is unchanged, not v1
v=120km/h=120000m/3600s=100/3m/s
The vehicle displacement in t = 0.5s is S1 = VT = 50 / 3 = 16.67m, V ^ 2 = 2as2, S2 = V1 ^ 2 / 2a, S2 = (1250 / 9) = 138.89m
S=S1+S2=156m
To score: S = 50 / 3 + 1250 / 9 = (1400 / 9) M