The distance between a and B is 90 kilometers. How many kilometers does a car leave B after driving from a to B at the speed of 80 kilometers per hour

The distance between a and B is 90 kilometers. How many kilometers does a car leave B after driving from a to B at the speed of 80 kilometers per hour

90 - 80 × 3 / 4 = 30 km
30 kilometers to go

The car goes uphill from place a to place B at a speed of 60 km / h, and goes downhill from place B to place a at a speed of 80 km / h
What's the average speed of the car back and forth

Average speed = 1 × 2 ^ (1 / 60 + 1 / 80) = 480 / 7 km / h

The highway from city a to city B is 160 km long. The speed of a car on the expressway is 80 km / h, and that on the ordinary highway is 40 km / h. how much time does it save to take the expressway from city a to city B?

160 △ 40-160 △ 80, = 4-2, = 2 (hours); a: Taking the expressway from city a to city B saves 2 hours compared with the ordinary highway