Question 323714
Since the distance to the warehouse is not given,
it can be any distance, and it doesn't change the problem
I'll say the distance is {{{40}}} mi
Let {{{t}}} = his time to drive back 
Ave speed is (total distance)/(total time)
The total distance is {{{2*40 = 80}}} mi
The time driving to the warehouse is {{{40/20 = 2}}} hrs
Ave speed = {{{80/(2 + t)}}}
{{{40 = 80/(2 + t)}}}
{{{40*(2 + t) = 80}}}
{{{2 + t = 2}}}
{{{t = 0}}}
My result is that he can't do it
If he was to average {{{39}}} mi/hr for the whole trip,
{{{39*(2 + t) = 80}}}
{{{2 + t = 2.0513}}}
{{{t = .0513}}} hr
Speed returning = {{{40/.0513 = 780}}} mi/hr
If he was to average {{{25}}} mi/hr
{{{25*(2 + t) = 80}}}
{{{2 + t = 3.2}}}
{{{t = 1.2}}} hr
Speed returning = {{{40/1.2 = 33.33}}} mi/hr
So, an ave speed for the whole trip
needs to be around 25 or 30 mi/hr
Maybe there's a flaw in my logic,
but this is what I get