Question 1097143
Average speed is ALWAYS: 
[ distance traveled ] / [ time spent traveling ]
Let {{{ s }}} = his speed on uphill terrain
{{{ s + 3 }}} = his speed on level ground
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{{{ s*5 + ( s + 3 )*2 = 27 }}}
{{{ 5s + 2s + 6 = 27 }}}
{{{ 7s = 21 }}}
{{{ s = 3 }}} mi/hr
and
{{{ s + 3 = 6 }}} mi/hr
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His average speed on level ground was 6 mi/hr
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check:
{{{ s*5 + ( s+3 )*2 = 27 }}}
{{{ 3*5 + 6*2 = 27 }}}
{{{ 15 + 12 = 27 }}}
{{{ 27 = 27 }}}
OK