Xiao Ming starts from a and goes to B by bicycle at the speed of V (km / h). It takes t (H). It is known that a and B are 6 km apart Q: at 15:00, Xiao Ming starts from B and is going to return to a along the same road. If Xiao Ming wants to return to a before 15:20, what is the average speed of his bicycle per hour?

Xiao Ming starts from a and goes to B by bicycle at the speed of V (km / h). It takes t (H). It is known that a and B are 6 km apart Q: at 15:00, Xiao Ming starts from B and is going to return to a along the same road. If Xiao Ming wants to return to a before 15:20, what is the average speed of his bicycle per hour?

The distance between AB and ab is 6km, and they can return in 20 minutes, that is, 1 / 3 hour, so the speed is at least:
Speed = 6 / (1 / 3) = 18 km / h

Xiaoqiang goes for an outing by bike. The picture on the right shows the distance y (km) from home and the time x (hours)
Xiaoqiang goes for an outing by bike. The graph on the right shows the relationship between the distance y (km) from home and the time x (hours). Xiaoming leaves home at 9 and goes home at 15
Ask: when is Xiaoqiang 21km away from home? (write the calculation process)
This is the first picture

It can be seen from the figure that: in the CD and EF segments, the distance may be 21km. Let CD function be y = KX + B, substituting (11,15) (12,30) into k = 15, B = - 150, so y = 15x-150, substituting y = 21 into x = 171 / 15, let EF segment function be y = KX + B, substituting (13,30) (15,0) into k = - 15, B = 225, so y = - 15x + 225, y =