Question 430978
Let s = the speed in still water
Let c = the speed of the current
The speed on the upstream trip will be: s - c
The speed on the downstream trip will be: s + c
The total time required is the sum of the upstream and downstream times:
t = t_up + t_down = {{{d/(s-c) + d/(s+c)}}}
Given: t = 2.5 hr, s = 5c, d = 12 mi
Therefore we have the following expression for the total time:
{{{2.5 = 12/(5c-c) + 12/(5c+c)}}}
This simplifies to:
{{{2.5 = 12/4c + 12/6c}}}