Question

Лодка по течению реки прошла 42км за 3 целых одну вторую часа. За сколько лодка пройдёт это же расстояние против течения реки, если скорость реки 2 км в час?Срочно дам 10 балов

Answer 1

Ответ: за 5,25 часа

Пошаговое объяснение:

42÷3,5=12 км/час скорость по течению

12-4=8км/час скорость против теч

42÷8=5,25часа

Answer 2

Problem Analysis

We are given that a boat traveled a distance of 42 km downstream in 3 and 1/2 hours. We need to find out how long it will take for the boat to travel the same distance upstream, given that the river's speed is 2 km/h.

Solution

To solve this problem, we can use the formula: time = distance / speed.

Let's calculate the time it took for the boat to travel downstream: - Distance = 42 km - Speed of the boat = speed of the river + speed of the boat relative to the river - Speed of the river = 2 km/h - Speed of the boat relative to the river = unknown

We can set up the equation as follows: 3 and 1/2 hours = 42 km / (2 km/h + speed of the boat relative to the river).

To find the speed of the boat relative to the river, we need to solve for it in the equation above.

Calculation

Let's calculate the speed of the boat relative to the river:

3 and 1/2 hours = 42 km / (2 km/h + speed of the boat relative to the river)

To simplify the calculation, let's convert 3 and 1/2 hours to a fraction: 3 and 1/2 = 7/2.

7/2 = 42 km / (2 km/h + speed of the boat relative to the river)

To isolate the speed of the boat relative to the river, we can cross-multiply:

7/2 * (2 km/h + speed of the boat relative to the river) = 42 km

Simplifying further:

7 km/h + 7/2 * speed of the boat relative to the river = 42 km

Subtracting 7 km/h from both sides:

7/2 * speed of the boat relative to the river = 42 km - 7 km/h

7/2 * speed of the boat relative to the river = 35 km

Dividing both sides by 7/2:

speed of the boat relative to the river = 35 km / (7/2)

Simplifying:

speed of the boat relative to the river = 35 km * (2/7)

speed of the boat relative to the river = 10 km/h

Therefore, the speed of the boat relative to the river is 10 km/h.

Answer

To find out how long it will take for the boat to travel the same distance upstream, we can use the formula: time = distance / speed.

- Distance = 42 km - Speed of the boat = speed of the river - speed of the boat relative to the river - Speed of the river = 2 km/h - Speed of the boat relative to the river = 10 km/h

Using the formula, we can calculate the time it will take for the boat to travel upstream:

time = 42 km / (2 km/h - 10 km/h)

Simplifying:

time = 42 km / (-8 km/h)

Since the speed is negative, we can take the absolute value:

time = 42 km / 8 km/h

Simplifying further:

time = 5.25 hours

Therefore, it will take the boat 5.25 hours to travel the same distance upstream.

Note: The calculation assumes that the speed of the boat relative to the river remains constant throughout the journey.