Question 765293
Let {{{ x }}} = quarts of pure sodium needed
{{{ .65*50 = 32.5 }}} quarts of pure sodium in 65% solution
-----------------
{{{ ( x + 32.5 ) / ( x + 50 )  = .75 }}}
{{{ x + 32.5 = .75*( x + 50 ) }}}
{{{ x + 32.5 = .75x + 37.5 }}}
{{{ .25x = 5 }}}
{{{ x = 20 }}}
20 quarts of pure sodium are needed
check:
{{{ ( x + 32.5 ) / ( x + 50 )  = .75 }}}
{{{ ( 20 + 32.5 ) / ( 20 + 50 )  = .75 }}}
{{{ 52.5 / 70 = .75 }}}
{{{ 52.5 = .75*70 }}}
{{{ 52.5 = 52.5 }}}
OK