Question 871396
Eric's rate of painting is 
( 1 room painted ) / ( 9 hrs )
Together, their rate is:
( 1 room painted ) / ( 4 hrs )
-------------------------
Let Matt's rate of painting =
( 1 room painted ) / ( t hrs )
-------------------------
Add their rates to get their rate 
working together
{{{ 1/9 + 1/t = 1/4 }}}
Multiply both sides by {{{ 36t }}}
{{{ 4t + 36 = 9t }}}
{{{ 5t = 36 }}}
{{{ t = 7.2 }}}
Note that {{{ .2 }}} hrs = {{{ .2*60 = 12 }}} min
----------------
Matt can paint the room by himself in 7 hrs 12 min