Question 1041593
Note that fat is the part that is not lean
{{{ .1*800 = 80 }}} pounds of fat in 800 pounds of meat
Let {{{ x }}} = pounds of hamburger that is 25% fat 
that needs to be added
{{{ .25x }}} = pounds of fat in 25% fat hamburger
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Notice I switched from lean to fat in each case
( that way I get smaller numbers )
{{{ ( 80 + .25x ) / ( 800 + x ) = .15 }}}
{{{ 80 + .25x = .15*( 800 + x ) }}}
{{{ 80 + .25x = 120 + .15x }}}
{{{ .1x = 40 }}}
{{{ x = 400 }}}
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400 pounds of hamburger that is 75% lean ( 25% fat )
must be added
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check:
{{{ ( 80 + .25*400 ) / ( 800 + 400 ) = .15 }}}
{{{ ( 80 + 100 ) / 1200 = .15 }}}
{{{ 180 = .15*1200 }}}
{{{ 180 = 180 }}}
OK