Question 956520
Their rate of cleaning:
[ 1 house cleaned ] / [ 7 hrs ]
{{{ 1/7 }}}
William's rate of messing it up:
[ 1 house messed up ] / [ 2 hrs ]
{{{ -1/2 }}}
( the minus sign is needed because  this
is the opposite of cleaning the house )
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{{{ 1/t }}} = rate of getting messed up
{{{ 1/7 - 1/2 = 1/t }}}
Multiply both sides by {{{ 14t }}}
{{{ 2t - 7t = 14 }}}
{{{ -5t = 14 }}}
{{{ t = -2.8 }}}
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In 2.8 hrs, the house is messed up again