Rate problems can be tackled easily, if you follow a method.
A simple way to do Rate problems is to use the following formula:
X1/T1 + X2/T2 = J
X1= Time spent together by A
X2 = Time spent together by B
T1 = Time spent alone by A
T2 = Time spent alond by B
J= Fraction of job
Here, X1 =6, X2= 6, T1=10, T2 = unknown, J= 1 (full job)
=> T2 = 15
(This problem could be solved in about 30 second with this method).
This formula can be used for almost all rate problems, including the one that involves many people once you think through the problem and obtain the correct parameters to plug it into the above equation. In GMAT this will help in terms of time!
See another rate problem that is solved easily using this formula at http://www.gmatclub.com/phpbb/viewtopic.php?t=13304