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David rowed a boat upstream for three miles and then returned to point he started from. The entire journey took four hours. The speed of the stream is one mile per hour. Find David's speed in still water. (Hint: speed = distance ÷ time, upstream speed = speed of the boat – speed of the stream, and downstream speed = speed of the boat + speed of the stream) David's speed in still water is ____ miles per hour.

Respuesta :

To solve for the time it takes to travel a certain distance at a certain speed, use the formula,
                                     time = distance / speed

Given that the entire journey takes four hours, add up the time it takes David to row upstream with that of him travelling downstream. This is mathematically represented as,

                              total time = time upstream + time downstream

                                         4 = 3/(x - 1) + 3/(x +1)

Solving for x in the equation gives 2. Thus, David's speed in still water is 2 miles per hour. 

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