Question 728685
Average speed is ( total distance ) /(  total time )
Going up:
{{{ d = 10t[1] }}}
{{{ t[1] = d / 10 }}}
Going down:
{{{ d = 30t[2] }}}
{{{ t[2] = d / 30 }}}
-----------------
{{{ 2d / ( t[1] + t[2] ) = 2d / ( d/10 + d/30 ) }}}
{{{  2d / ( d/10 + d/30 ) = 2 / ( 1/10 + 1/30 ) }}}
{{{ 2 / ( 3/30 + 1/30 ) }}}
{{{ 2 / ( 4/30 ) = 60/4 }}}
{{{ 60/4 = 15 }}}
The average speed is 15 mi/hr