# Water current

John swims upstream. After a while, he passes the bottle, from that moment he floats for 20 minutes in the same direction. He then turns around and swims back, and from the first meeting with the bottle, he sails 2 kilometers before he reaches the bottle. What is the current speed? (John is still swimming at the same speed regardless of the current.

Result

f =  3 km/h

#### Solution:

$s_{1}=(j-f) \cdot \ 20/60 \ \\ s_{2}=2 \ \text{km} \ \\ s_{2}=f \cdot \ (\dfrac{ s_{1}+s_{2} }{ f+j } + 20/60) \ \\ 2=f \cdot \ (\dfrac{ (j-f) \cdot \ 20/60+2 }{ f+j } + 20/60) \ \\ 2=f \cdot \ (\dfrac{ (j-f)/3+2 }{ f+j } + 1/3) \ \\ \ \\ (f-3)j/ (f+j)=0 \ \\ \ \\ j,f>0 \ \\ j \ne f \ \\ \ \\ f-3=0 \ \\ f=3 \ \text{km/h}$

Our examples were largely sent or created by pupils and students themselves. Therefore, we would be pleased if you could send us any errors you found, spelling mistakes, or rephasing the example. Thank you!

Leave us a comment of this math problem and its solution (i.e. if it is still somewhat unclear...):

Be the first to comment!

Tips to related online calculators
Do you have a linear equation or system of equations and looking for its solution? Or do you have quadratic equation?
Do you want to convert velocity (speed) units?
Do you want to convert time units like minutes to seconds?

## Next similar math problems:

1. Filament of bulb
The filament of bulb has a 1 ohm resistivity and is connected to a voltage 220 V. How much electric charge will pass through the fiber when the electric current passes for 10 seconds?
2. Resistance of the resistor
The resistor terminals have a voltage of 20 V and a current of 5 mA is passed through. What is the resistance of the resistor?
3. Coil as a girl
The electrical resistance of the copper wire coil is 2.0 ohms. What current runs through the coil when the voltage between the terminals is 3.0 V?
4. Resistance
Determine the resistance of the bulb with current 200 mA and is in regular lamp (230V).
5. Fog
The car started in fog at speed 30 km/h. After a 12-minute drive, the fog dissipated and the driver drove next 12 minutes distance 17 km. On the last 17 km long again the driving conditions deteriorated and the driver drove the speed of 51 km/h. a) Calcu
6. Aircraft angines
The two engines of the aircraft are enough to supply the fuel for five hours of operation. However, one of the engines has a malfunction and thus consumes one-third more fuel. How long can the plane be in the air before it runs out of fuel? After an hour
7. Cyclists and walkers
A group of tourists started at 8:00 at speed 4 km/h walk. At half-past ten, another group started on a bike and caught up with a group of tourists at 10:50. What was the average speed of cyclists?
8. Where and when
The truck left Kremnica at 11:00 h at a speed of 60km/h. At 12:30 h, the passenger car started at an average speed of 80km/h. How many kilometers from Kremnica will the passenger car reach truck, and when?
9. Car loop
The toy car runs at an average speed of 2 m/s. In a minute, it will pass the entire circuit five times. How long is the loop?
10. Transformer
Solve the textbook problems - transformer: a) N1 = 40, N2 = 80, U2 = 80 V, U1 =? b) N1 = 400, U1 = 200 V, U2 = 50 V, N2 =?
11. Cheetah vs antelope
When the cheetah began chasing the antelope, the distance between them was 120 meters. Although the antelope was running at 72km/h, the cheetah caught up with it in 12 seconds. What speed was the cheetah running?
12. Voltmeter range
We have a voltmeter which in the original set measures voltage to 10V. Calculate the size of the ballast resistor for this voltmeter, if we want to measure the voltage up to 50V. Voltmeter's internal resistance is 2 kiloohm/Volt.
13. The tourist
The tourist traveled 78km in 3 hours. Part of the journey went on foot at 6km/h, the rest of the trip by bus at 30km/h. How long did he walk?
14. Between two bus stops
Wanda lives between two bus stops at three-eighths of their distance. He started the house today and found that whether he was running to one or the other stop, he would have arrived at the bus stop. The average bus speed is 60 km/h. What is the average
15. RPM
An electric motor makes 3,000 revolutions per minutes. How many degrees does it rotate in one second?
16. Gravitation
From the top of the 80m high tower, the body is thrown horizontally with an initial speed of 15 m/s. At what time and at what distance from the foot of the tower does the body hit the horizontal surface of the Earth? (use g = 10 ms-2)
17. Rocket start
The body launched vertically up returns to the start site in 6 seconds. What height did it have?