Question

Из двух городов, расстояние между которыми 1680км вышли одновременно на встречу друг другу два поезда. Первый поезд проходит это растояние 21ч, а второй за 28ч. Через сколько часов они встречаться?

Answer 1

1)1680:21=80 (км/ч)-скорость 1 поезда.
2)1680:28=60 (км/ч)-скорость 2 поезда.
3)1680:(60+80)=12 (ч.)-будет встреча.
Ответ: через 12часов

Answer 2

Problem Analysis

We have two trains traveling towards each other from two cities that are 1680 km apart. The first train takes 21 hours to cover this distance, while the second train takes 28 hours. We need to determine how many hours it will take for the trains to meet.

Solution

To find the time it takes for the trains to meet, we can use the concept of relative speed. The relative speed of the two trains is the sum of their individual speeds when they are moving towards each other.

Let's assume the speed of the first train is x km/h and the speed of the second train is y km/h.

We know that the distance covered by the first train is 1680 km and it takes 21 hours. Therefore, we can write the equation:

Distance = Speed × Time

For the first train: 1680 = x × 21

Simplifying the equation, we find: x = 1680 / 21 x = 80 km/h

Similarly, for the second train: 1680 = y × 28

Simplifying the equation, we find: y = 1680 / 28 y = 60 km/h

Now that we have the speeds of both trains, we can find their relative speed by adding their individual speeds: Relative speed = x + y Relative speed = 80 + 60 Relative speed = 140 km/h

To find the time it takes for the trains to meet, we can use the formula: Time = Distance / Relative speed

Substituting the values, we find: Time = 1680 / 140 Time = 12 hours

Therefore, the two trains will meet after 12 hours.

Answer

The two trains will meet after 12 hours.