The distance between a and B is 31 km. A goes from a to B by bike. An hour later, B goes from a to B by motorcycle. It is known that a is 12 km / h and B is 28 km / h ① Ask B how many hours after starting to catch up with a; 2. If B returns immediately after arriving at B, how long is the distance between B and a on the way back PS: not much

The distance between a and B is 31 km. A goes from a to B by bike. An hour later, B goes from a to B by motorcycle. It is known that a is 12 km / h and B is 28 km / h ① Ask B how many hours after starting to catch up with a; 2. If B returns immediately after arriving at B, how long is the distance between B and a on the way back PS: not much

(1) Let B catch up with a in X hours after departure,
28x=12+12x
∴x=3/4
(2) Let B meet a on the way back y hours after departure,
28x-31+12*(x+1)=31
∴x=5/4

The distance between a and B is 30 km. A goes from a to B by bike. An hour later, B goes from a to B by motorcycle. It is known that a is 12 km / h and B is 28 km / h
If B returns immediately after arriving at B, how long does it take for B to leave when he meets a on the way back

A walked 12x1 = 12km first
Let b set out and meet in X hours
12x+12+28x=30*2
40x=60-12
40x=48
x=1.2

It takes 4 minutes for a train to pass a bridge with a length of 2.7 km. It is known that the speed of the train is 0.8 km per minute. How long is the train?

8 × 4-2.7, = 3.2-2.7, = 0.5 (km), = 500 m; answer: the length of the train body is 500 m

A bridge is 2400 meters long. A train passes through the bridge at the speed of 900 meters per minute. It takes three minutes from the front of the train to the rear of the train to leave the bridge______ Rice

3 × 900-2400 = 2700-2400, = 300 (m); answer: the train body is 300 m long