Question 851396
Let {{{ x }}} = ml of 24% lemon juice solution needed
{{{ .24x }}} = ml of lemon juice in 24% solution 
{{{ .02*80 = 1.6 }}} ml of lemon juice in 2% solution
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{{{ ( .24x + 1.6 ) / ( x + 80 ) = .04 }}}
{{{ .24x + 1.6 = .04*( x + 80 ) }}}
{{{ .24x + 1.6 = .04x + 3.2 }}}
{{{ .2x = 1.6 }}}
{{{ x = 8 }}}
8 ml of 24% lemon juice solution are needed
check:
{{{ ( .24x + 1.6 ) / ( x + 80 ) = .04 }}}
{{{ ( .24*8 + 1.6 ) / ( 8 + 80 ) = .04 }}}
{{{ ( 1.92 + 1.6 ) / 88 = .04 }}}
{{{ 3.52 / 88 = .04 }}}
{{{ 3.52 = .04*88 }}}
{{{ 3.52 = 3.52 }}}
OK