Question 877874
Let {{{ a }}} = liters of pure acid needed
{{{ .1*4 = .4 }}} liters of acid in 10% solution
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{{{ ( a + .4 ) / ( a + 4 )  = .8 }}}
{{{ a + .4 = .8*( a + 4 ) }}}
{{{ a + .4 = .8a + 3.2 }}}
{{{ .2a = 2.8 }}}
{{{ a = 14 }}}
14 liters of pure acid should be added
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check:
{{{ ( a + .4 ) / ( a + 4 )  = .8 }}}
{{{ ( 14 + .4 ) / ( 14 + 4 )  = .8 }}}
{{{ 14.4 / 18 = .8 }}}
{{{ 14.4 = .8*18 }}}
{{{ 14.4 = 14.4 }}}
OK