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The distance between port a and port B is 1200 km. Ship a sails from port a to port B at 60 km per hour. 30 minutes after the departure of ship a, ship B sails from port B to port a at 70 km per hour. How many hours after the departure of ship B?
30 minutes = 12 hours, (1200-60 × 12) △ 60 + 70, = 1170 △ 130, = 9 hours, a: the ship B will meet 9 hours after departure
When the ship arrives at port B along the water, it goes back against the water. It takes 8 o'clock to know that the forward speed is 20 km faster than the reverse speed, and the first 4 o'clock is 60 km more than the last 4 o'clock?
Let v be the downstream speed, v-20 be the upstream speed, t be the downstream arrival time, t be the upstream time, and t be 8-t. the first 4:00 is 60 km more than the last 4:00. It can be seen that the downstream time must be less than the upstream time, t must be less than 4 hours, so there are both downstream time t and upstream time in the first 4 hours
When the ship arrives at port B downstream, it goes back upstream. It takes 8 hours to know that the forward speed is 20 km faster than the reverse speed per hour, and the first 4 hours is 60 km more than the last 4 hours
Because it's fast to go downstream and slow to go backward, it doesn't take four hours to go downstream, but it takes more than four hours to go upstream. In the first four hours, it's sure that it's all finished downstream, and part of the time is driving upstream. The speed of driving upstream is the same, and the distance of driving at the same time is the same. Therefore, it's necessary to drive 60 kilometers more in the first four hours than in the next four hours, That is to say, we can travel 60 kilometers more along the water than against the water in the same time;
The speed of driving along the water is 20 km faster than that of driving against the water, and the time required to complete more than 60 km is 60 △ 20 = 3 hours. It is concluded that the driving time of two ports along the water is 3 hours, and the driving against the water is 8-3 = 5 hours, then the distance between the two ports is s = 20 △ 1 / 3-1 / 5 = 150 km
The distance between port a and port B is 1200km. Ship a sails from port a to port B, traveling 60km per hour. 30 minutes after the departure of ship a, ship B sails from port B to port a, traveling 70km per hour. How many hours after the departure of ship B, the two ships meet?
(1200-60 × 0.5) / (60 + 70) = 9 hours
A: the two ships met nine hours after the departure of the second ship
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