Question 1137246
Let {{{ x }}} = reduction in cm of length and width
{{{ ( 15 - x )*( 9 - x )*2 = .8*15*9*2 }}}
{{{ 135 - 24x + x^2  = 108 }}}
{{{ x^2 - 24x  = 108 - 135 }}}
{{{ x^2 - 24x + 27 = 0 }}}
{{{ x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
{{{ x = ( -(-24) +- sqrt( (-24)^2 - 4*1*27 ))/(2*1) }}}
{{{  x = ( 24 +- sqrt( 576 - 108 ))/2 }}}
{{{ x = ( 24 + sqrt( 468 ))/2 }}}
{{{ x = ( 24 + 21.6333 )/2 }}} ( too large )
{{{ x = ( 24 - 21.6333 )/2 }}}
{{{ x = 2.3667/2 }}}
{{{ x = 1.18335 }}}
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Length = 13.8167
Width = 7.81665
thickness = 2 
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check:
{{{ 13.8167*7.81665*2 = 216 }}}
{{{ .8*15*9*2 = 216 }}}
OK