Question 958982
Let a, b, c, be the times for each to do 1 job if each alone.


{{{system(1/a+1/b+1/c=1/(a-6),1/a+1/b+1/c=1/(b-1),1/a+1/b+1/c=1/(c/2))}}}
Three equations in three unknown variables.  The question is to find the reciprocal of {{{1/a+1/b}}}.


Could you try using the last equation to eliminate c, and have or make a simpler system?  I have not worked through this; only setup the system.