Question 1156588
Let {{{ x }}} = ounces of 20% alcohol needed
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{{{ ( .2x + .25*13 ) / ( x + 13 ) = .21 }}}
{{{ ( .2x + 3.25 ) / ( x + 13 ) = .21 }}}
{{{ .2x + 3.25 = .21*( x + 13 ) }}}
{{{ .2x + 3.25 = .21x + 2.73 }}}
{{{ .01x = .52 }}}
{{{ x = 52 }}}
52 ounces of 20% alcohol are needed
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check:
{{{ ( .2x + .25*13 ) / ( x + 13 ) = .21 }}}
{{{ ( .2*52 + .25*13 ) / ( 52 + 13 ) = .21 }}}
{{{ ( 10.4 + 3.25 ) / 65 = .21 }}}
{{{ 10.4 + 3.25 = 13.65 }}}
{{{ 13.65 = 13.65 }}}
OK