Question 914160
( average rate ) = ( total distance ) / ( total time ) 
Let {{{ d }}} = total distance traveled
{{{ 21 = d / 13 }}}
{{{ d = 13*21 }}}
{{{ d = 273 }}} mi
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The distance from the mountains is 1/2 the 
total distance
{{{ 273/2 = 136.5 }}}
Alan lives 136.5 miles from the mountains