Question

If it takes you 40 minutes to go 20 miles downstream and then 60 minutes on the way back, what is the speed of the current?

Answer 1

Speed of Current = 5 miles per hour

Explanation:

We know distance formula,

D = RT

Where

D is distance

R is rate

T is time

If we let speed of boat (assume) to be "x" and speed of current to be "c"

Then downstream rate is (with current) = x + c

Upstream rate is (against current) = x - c

40 mins to go 20 miles downstream, that means:

D = RT

20 = (x + c)(40)

and

60 minutes to go upstream, 20 miles, that means:

D = RT

20 = (x - c)(60)

Simplifying first equation:

40x + 40c = 20

Simplifying second equation:

60x - 60c = 20

Multiplying first equation by 60, we get:

60 * [40x + 40c = 20] = 2400x + 2400c = 1200

Multiplying second equation by 40, we get:

40 * [60x - 60c = 20] = 2400x - 2400c = 800

Now we add up both these equations:

2400x + 2400c = 1200

2400x - 2400c = 800

----------------------------------

4800x = 2000

x = 2000/4800 = 5/12

We need speed of current, that is "c", so we plug in the value of x into first equation and solve for c:

`40x + 40c = 20\\40((5)/(12)) + 40c = 20\\(50)/(3)+40c=20\\40c=20-(50)/(3)\\40c=(10)/(3)\\c=(1)/(12)`

Speed of Current = 1/12 miles per minute

Since there is 60 minutes in an hours, that would be:

(1/12) * 60 = 5 miles per hour

Speed of Current = 5 miles per hour