Question 25642
Work done = (rate)(time)
Mary work along: =(1/10 job/hr.)(2 hrs.)=1/5 job
Now let the time "they" work together be "x".
Note the phrase "...take them to finish..." can 
have two meanings.
It can mean Bill and Sue finished the job, or
it can mean Mary, Bill, and Sue finished the job.
If the 1st you get the following equation:
(1/8)x+(1/12)x=4/5
15x+10x=96
    27x=96
      x=96/27 hrs.  (time Bill and Sue work together to finish the job)
If all three work together for "x" hours:
   (1/8)x+(1/12)x+(1/10)x=4/5
     15x+10x+12x=96
            37x=96
              x=96/37 hrs (time Bill and Sue and Mary work together)

Cheers,
Stan H.