Question 909135
x - speed of the boat in still water
x-5 - speed of the boat upstream 
x+5 - speed of the boat downstream 

{{{speed=(distance)/(time)}}}
Downstream:{{{x+5=18/t}}}
 {{{t=18/(x+5)}}}
since time is the same,
Upstream:
{{{x-5=8/(18/(x+5))}}}
{{{x=13}}}