Question 852414
This part of your description is bad:  "a constant speed of 20 miles per hr relative to the water".


The trip distance is also not perfectly clear; I'm assuming as 24 miles in either direction.

Try to put data into a table.


__________________speed________time_______distance
Upstream__________20-c_________(___)______24
Downstream________20+c_________(___)______24
TOTAL__________________________2.5


Uniform rate for travel follows {{{RT=D}}}, so fill the time information as {{{T=D/R}}}.


__________________speed________time_______distance
Upstream__________20-c_________{{{24/(20-c)}}}______24
Downstream________20+c_________{{{24/(20+c)}}}______24
TOTAL__________________________2.5


The time sum equation has only the variable, c as unknown.  That is the speed of the current of the river.
{{{highlight_green(24/(20-c)+24/(20+c)=2.5)}}}.


SOLVING THE EQUATION
LCD is (20-c)(20+c).
{{{24(20+c)+24(20-c)=2.5(20^2-c^2)}}}
{{{24(20+c+20-c)=(5/2)(400-c^2)}}}
{{{24*20*2=5(400-c^2)}}}
{{{720=2000-500c^2}}}
{{{720-2000=-500c^2}}}
{{{500c^2=2000-720}}}
{{{500c^2=1280}}}
{{{50c^2=128}}}
{{{c^2=128/50=2.56}}}
{{{highlight(c=1.6)}}}