Question 908325
From 1 PM to 2 PM is 1 hr
------------------------
Bob's rate of cleaning is:
( 1 room ) / ( 10 hrs ) = {{{ 1/10 }}}
--------------------
The fraction of the room cleaned by
Bob in {{{ 1 }}} hr is:
{{{ 1*( 1/10 ) = 1/10 }}} of the room
-------------------------------
That means there is {{{ 9/10 }}} of the 
room left to clean
-----------------
Add Bob's rate and his Mom's rate to 
get their rate working together:
Let {{{ t }}} = time in hrs for them to
finish working together
{{{ 1/10 + 1/5 = (( 9/10)) / t }}}
Multiply both sides by {{{ 10t }}}
{{{ t + 2t = 9 }}}
{{{ 3t = 9 }}}
{{{ t = 3 }}}
-------------
3 hrs after 2 PM is 5 PM
They will finish cleaning the room at 5 PM