Question 270018
You don't take an average of 2 averages to get
an overall average. The correct way to do it  is:
(total distance travelled)/(total time for trips) = average speed
Let {{{t}}} = his time in hrs for return trip
{{{(30 + 30)/(3 + t) = 15}}} mi/hr
Multiply both sides by {{{3 + t}}}
{{{60 = 15*(3 + t)}}}
{{{60 = 45 + 15t}}}
{{{15t = 15}}}
{{{t = 1}}} hr
It will take him 1 hr to make the 30 mi return trip, so
return speed  = {{{30/1 = 30}}} mi/hr
check answer:
{{{(30 + 30)/(3 + 1) = 15}}}
{{{60/4 = 15}}}
{{{15 = 15}}}
OK