SOLUTION: Al travels in his motorboat downstream 12 miles with the current. He then turns around and heads back to the starting point going upstream against the current. The total time of hi

Let "u" be the boat speed in still water, in miles per hour.


Then the boat' speed downstream is (u+2) miles per hour, while

     the boat' speed downstream is (u-2) miles per hour.


Time to travel downstream is  hours, while

time to travel upstream is   hours.


The total time is   +   hours.


According to the condition, it is 5 hours, which gives you an equation

     +  = 5.


To solve it, multiply both sides by (u+2)*(u-2) = (u^2-4).  You will get


    12*(u-2) + 12*(u+2) = 5*(u^2-4)


    12u - 24 + 12u + 24 = 5u^2 - 20,

    5u^2 - 24u - 20 = 0


     =  = .


Only positive root works  u =  = 5.524.


Answer.  The boat' speed in still water is 5.524 miles per hour.


Check.    = 5.00 hours.   ! Correct !