<PRE><B>Question 548143</B>
First find the fraction of the room that 
person cleans from 1 PM to 2 PM
Person&#39;s rate = ( 1 room ) / ( 10 hrs )
{{{ (1/10)*1 = 1/10 }}}
Person cleans 1/10 of the room in 1 hr
There is 9/10 of the room left to clean
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Add their rates.
Let {{{t}}} = time for both to finish room
With person and Mother both cleaning:
{{{ ( 1/10 ) + ( 1/5 ) = (9/10) / t }}}
Multiply both sides by {{{10}}}
{{{ 1 + 2 = 9/t }}}
{{{ t = 9/3 }}}
{{{ t = 3 }}} hrs
The room will be clean in {{{ 1 + 3 = 4 }}} hrs
4 hrs from 1 pm is 5 pm


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