Question 774903
Let {{{ x }}} = liters of pure acid to be added
{{{ .25*20 = 5 }}} liters of acid in 25% solution
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{{{ ( 5 + x ) / ( 20 + x ) = .4 }}}
{{{ 5 + x = .4*( 20 + x ) }}}
{{{ 5 + x = 8 + .4x }}}
{{{ .6x = 8 - 5 }}}
{{{ .6x = 3 }}}
{{{ x = 5 }}}
5  liters of pure acid need to be added
check:
{{{ ( 5 + x ) / ( 20 + x ) = .4 }}}
{{{ ( 5 + 5 ) / ( 20 + 5 ) = .4 }}}
{{{ 10 / 25 = .4 }}}
{{{ 10 = .4*25 }}}
{{{ 10 = 10 }}}
OK