Question 610660
First find out what fraction of the job Jeff
and his Wife did by themselves
Add their rates of working to get their rate 
working together.
( 1 garden/ 4 hrs ) + ( 1 garden / 4 hrs ) = x / 1 
Where {{{x}}} is the fraction of the job they do in {{{1}}} hr
{{{ 1/4 + 1/4 = x/1 }}}
{{{ x = 2/4 }}}
{{{ x = 1/2 }}}
So, they get 1/2 of the job done in 1 hr
There is 1/2 the job left to be done. Then Rory joins them
for 1/2 hr
Add all their rates of working
{{{ 1/4 + 1/4 + R = 1/( 1 + 1/2 ) }}}
Where {{{ R }}} is Rory's rate of working
{{{ 1/2 + R = 1/((3/2)) }}}
{{{ 1/2 + R = 2/3 }}}
{{{ R = 2/3 - 1/2 }}}
{{{ R = 4/6 - 3/6 }}}
{{{ R = 1/6 }}}
So, Rory can do ( 1 job ) / ( 6 hrs )
So, he can get the whole job done by himself
in 6 hrs.
------------
check:
{{{ 1/2 + R = 1/((3/2)) }}}
{{{ 1/2 + 1/6 = 2/3 }}}
{{{ 3/6 + 1/6 = 4/6 }}}
{{{ 4/6 = 4/6 }}}
OK