Question

❗️ПОМОГИТЕ СРОЧНО 4 класс❗️ Из двух городов, удаленных друг от друга на 1680 км, вышли одновременно навстречу друг другу два поезда. Первый поезд проходит это расстояние за 21 час, а второй – за 28 ч. Через сколько часов поезда встретятся?

Answer 1

1)1680:21=80(км/ч)-скорость первого

2)1680:28=60(км/ч)-скорость второго

3)80+60=140(км/ч)-скорость сближения

4)1680:140=12(ч)

Ответ: через 12 часов они встретятся

Answer 2

1) 1680:21=80(км/ч)-скорость первого поезда.

2) 1680:28=60(км/ч)-скорость второго поезда.

3) 80+60=140(км) -скорость сближения

4) 1680:140=12(ч) -будут ехать поезда до встречи.

Ответ:через 12 часов.

Answer 3

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 solve this problem, we can use the concept of relative speed. The relative speed of two objects moving towards each other is the sum of their individual speeds. We can calculate the relative speed of the two trains and then use it to find the time it takes for them to meet.

Let's denote the speed of the first train as v1 and the speed of the second train as v2. Since the distance traveled by both trains is the same (1680 km), we can set up the following equation:

Distance = Speed × Time

For the first train: Distance = v1 × 21

For the second train: Distance = v2 × 28

Since the trains are moving towards each other, their combined distance is equal to the total distance between the two cities (1680 km):

v1 × 21 + v2 × 28 = 1680

We need to find the time it takes for the trains to meet, which we can denote as t. The distance traveled by the first train in time t is v1 × t, and the distance traveled by the second train in time t is v2 × t. Therefore, the equation becomes:

v1 × t + v2 × t = 1680

Factoring out t:

t × (v1 + v2) = 1680

Finally, we can solve for t:

t = 1680 / (v1 + v2)

Let's calculate the value of t using the given speeds of the trains.

Calculation

Given that the first train takes 21 hours to cover 1680 km, we can calculate its speed as:

v1 = Distance / Time = 1680 km / 21 hours = 80 km/h

Similarly, the speed of the second train is:

v2 = Distance / Time = 1680 km / 28 hours = 60 km/h

Now, we can substitute these values into the equation to find the time it takes for the trains to meet:

t = 1680 km / (80 km/h + 60 km/h) = 1680 km / 140 km/h = 12 hours

Answer

The two trains will meet after 12 hours.

Note: The answer provided is based on the information given in the question. If there are any additional factors or conditions that need to be considered, please provide them for a more accurate response.