Question 942085
average speed is always:
( total distance ) / ( total time )
{{{ d = 20*( 12/60 ) }}}
{{{ d = 4 }}} mi
{{{ t[1]= 3/15}}}
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{{{ d = 60t }}}
{{{ 4 = 60t }}}
{{{ t[2] = 4/60 }}}
{{{ t[2] = 1/15 }}} hrs
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Ave speed ={{{  2*4 / ( 3/15 + 1/15 ) }}}
Ave speed= {{{ 8 / (4/15) }}}
Ave speed = {{{ ( 15/4 )*8 }}}
Ave speed = {{{ 30 }}} 
Average speed to the store and back is 30 mi/hr