work=speed times time (my equation to help me figure it out)
ok, 10 men take 84 days to complete a job
therefor, each man completes 1/10 of the job in 84 days
each man does 1/840 of the job per 1 day
so, if we had 5 more, or 15 men
15 times 1/840 times xdays=1 job complete
15/840 times xdays=1job
times both sides by 840/15
xdays=840/15
xdays=56
56 days it would take