Question 568850
Let {{{ x }}} = ounces of 90% solution needed
{{{ .9x }}} = ounces of alcohol in 90% solution
{{{ .5*5 = 2.5 }}} ounces of alcohol in 50% solution
given:
{{{ ( .9x + 2.5 ) / ( x + 5 ) = .8 }}}
{{{ .9x + 2.5 = .8*( x + 5 ) }}}
{{{ .9x + 2.5 = .8x + 4 }}}
{{{ .1x = 1.5 }}}
{{{ x = 15 }}}
15 ounces of 90% solution are needed
check answer:
{{{ ( .9*15 + 2.5 ) / ( 15 + 5 ) = .8 }}} 
{{{ ( 13.5 + 2.5 ) / 20 = .8 }}}
{{{ 16 = .8*20 }}}
{{{ 16 = 16 }}}
OK