SOLUTION: The current in a stream moves at a speed of 4mi/h. A boat travels 4 miles upstream and 28 km downstream in a total time of 4 hours. What is the speed of the stream?

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Question 442824: The current in a stream moves at a speed of 4mi/h. A boat travels 4 miles upstream and 28 km downstream in a total time of 4 hours. What is the speed of the stream?
Answer by jorel1380(3719)   (Show Source): You can put this solution on YOUR website!
4/x-4+28/x+4=4
4(x+4)+28(x-4)=4(x2-16)
4x+16+28x-112=4x2-64
0=4x2-32x+32
0=x2-8x+8
Solved by pluggable solver: SOLVE quadratic equation with variable
Quadratic equation (in our case ) has the following solutons:



For these solutions to exist, the discriminant should not be a negative number.

First, we need to compute the discriminant : .

Discriminant d=32 is greater than zero. That means that there are two solutions: .




Quadratic expression can be factored:

Again, the answer is: 6.82842712474619, 1.17157287525381. Here's your graph:

Throwing out the lower number, we get the speed of the boat in still water to be 6.828 mph..
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