Question 981444
Let {{{ a }}} = ml of 15% solution needed
{{{ .15a }}} = ml of alcohol in 15% solution
{{{ .85*80 = 68 }}} ml of alcohol in 85% solution
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{{{ ( .15a + 68 ) / ( a + 80 ) = .4 }}}
{{{ .15a + 68 = .4*( a + 80 ) }}}
{{{ .15a + 68 = .4a + 32 }}}
{{{ .25a = 36 }}}
{{{ a = 144 }}}
144 ml of 15% solution are needed
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check:
{{{ ( .15a + 68 ) / ( a + 80 ) = .4 }}}
{{{ ( .15*144 + 68 ) / ( 144 + 80 ) = .4 }}}
{{{ ( 21.6 + 68 ) / 224 = .4 }}}
{{{ 89.6 = .4*224 }}}
{{{ 89.6 = 89.6 }}}
OK