SOLUTION: A boat can travel 39mph in still water. If it travels 235 miles with the current in the same length of time it travels 155 miles against the current, what is the speed of the curre

The time equation is


     = .


Cancel factor 5 in both sides


     = .


Obviously,  x = 8 is the solution.


ANSWER.  8 miles per hour.

    such a speed usually is considered as too high for a river to travel on it by boat.

    The speed of a river varies from close to 0 m/s to 3.1 m/s (7 mph). Factors that affect the speed of a river 
    include the slope gradient, the roughness of the channel, and tides. Rivers tend to flow from a higher elevation 
    to a lower elevation. The gradient is the drop of the elevation of a river. 

    Therefore, the rivers speed is at its maximum at the headwaters (high gradient, high energy) 
    and at its minimum at the base level (no gradient, lowest energy). 
    An incoming tide can reverse a river and cause it to flow against the gradient -- uphill!

Let speed of current be C

Then we get the following TIME equation: 

 ---- Factoring out, GCF, 5, in numerator

31(39 + C) = 47(39 - C) ------ Cross-multiplying

31(39) + 31C = 47(39) - 47C

31C + 47C = 47(39) - 31(39)

78C = 16(39)

Speed of current, or