Question 820743
ratio L:W
{{{3:1}}} ={{{ 3/1 }}}
{{{ 12:W }}}={{{ 12/W}}}
 
equate the two ratio

{{{ 3/1=12/W}}}
simplify
{{{ 3W=12}}}
{{{ 3W/3=12/3}}}
{{{ W=4 }}}