Question 1180672
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The numbers look unreasonable.  Only 3km upstream and back in 4.5 hours means the speed of the stream must be nearly equal to the speed of the boat.<br>
In any case....<br>
x = speed of stream
15+x = downstream speed
15-x = upstream speed
3/(15+x) = downstream time
3/(15-x) = upstream time<br>
The total time is 4.5 hours:<br>
{{{3/(15+x)+3/(15-x)=4.5}}}<br>
With the suspicious numbers, I wouldn't even try solving that algebraically.  I would graph the two functions {{{3/(15+x)+3/(15-x)}}} and {{{4.5}}} on a graphing calculator and find where they intersect.<br>
{{{graph(400,400,-1,15,-1,5,3/(15+x)+3/(15-x),4.5)}}}<br>