Question 719910
Let {{{ x }}} = gallons of water to be removed and
replaced with pure water
The owner starts out with {{{ .02*50 = 1 }}} gallon of the chemical
The gallons of chemical that are removed when the water is drained
is {{{ .02x }}}
Note that there are 50 gallons at the start and 50 gallons at the end
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{{{ ( 1 - .02x ) / 50 = .0125 }}} 
{{{ 1 - .02x = .0125*50 }}}
{{{ 1 - .02x = .625 }}}
{{{ .02x = 1 - .625 }}}
{{{ .02x = .375 }}}
{{{ x = 18.75 }}}
18.75 gallons should be drained and replaced with pure water
check:
{{{ ( 1 - .02*18.75 ) / 50 = .0125 }}} 
{{{ ( 1 - .375 ) / 50 = .0125 }}}
{{{ .625 / 50 = .0125 }}}
{{{ .625 = .625 }}}
OK