Question 512601
Average speed is ( total distance ) / ( total time )
For the 1st 6 miles
{{{ 6 = 4*t }}}
{{{ t = 1.5 }}} hrs
Let {{{ s }}} = speed for last {{{ 2.5 }}} hrs
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Average speed = {{{ 6 }}} mi/hr
Total distance = {{{ 6 + 2.5s }}}
{{{ ( 6 + 2.5s ) / ( 1.5 + 2.5 ) = 6 }}}
{{{ ( 6 + 2.5s ) / 4 = 6 }}} 
{{{ 6 + 2.5s = 24 }}}
{{{ 2.5s = 18 }}}
{{{ s = 7.2 }}}
He would have to travel at 7.2 mi/hr