Question 1005175
Let {{{ x }}} = liters of water to be added
{{{ .2*100 = 20 }}} liters of water in 100 liters
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{{{ ( x + 20 ) / ( x + 100 ) = 1/3 }}}
Multiply both sides by {{{ 3*( x + 100 ) }}}
{{{ 3*( x+20 ) = x + 100 }}}
{{{ 3x + 60 = x + 100 }}}
{{{ 2x = 40 }}}
{{{ x = 20 }}}
20 liters of water must be added
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check:
{{{ ( x + 20 ) / ( x + 100 ) = 1/3 }}}
{{{ ( 20 + 20 ) / ( 20 + 100 ) = 1/3 }}}
{{{ 40 / 120 = 1/3 }}}
{{{ 1/3 = 1/3 }}}
OK