Well, if the first worker can get it done on her own in eight days, then I'd be surprised to find that the group of them working together will take ten days to complete it. Let's look at work rates:
Worker A does the work at 1 job per 8 days which is one-eighth of a job per day.
Worker B does the work at 1 job per 10 days which is one-tenth of a job per day.
Worker C does the work at 1 job per 12 days which is one-twelfth of a job per day.
If you add the Rates, then you can get a team Rate:
1/8 + 1/10 + 1/12 = 15/120 + 12/120 + 10/120 = 37/120
That means they work at a rate that would get 37 jobs done in 120 days. Can you use this to figure out how many days it would take to get one job done?
Hope this helps,
Stephen La Rocque>