Question 1010509
Let {{{ x }}} = litters of solution to be drained off
and replaced with pure chocolate
{{{ .1*x }}} = litters of chocolate in solution drained off
{{{ .1*35 = 3.5 }}} litters of chocolate to begin with
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{{{ ( 3.5 - .1x + x ) /  35  = .45 }}}
{{{ 3.5 - .1*x + x = .45*35 }}}
{{{ 3.5 - .1*x + x = 15.75 }}}
{{{ .9x = 12.25 }}}
{{{ x = 13.611 }}}
13.611 litters must be drained off and
replaced with pure chocolate