At the end of 90 days, 90 /200 = 9/20 of the job was finished
Then 10 workers were fired so, 20 remain
Notice that if 30 workers can finish the job in 200 days, then each day 1 worker can complete 1 / (30 * 200) = 1/6000 of the job
So 20 workers in one day can complete 20/6000 = 1/300 of the job....so in 30 days, they can complete another
30 / 300 = 1/10 of the job = 2/20 of the job
So before the last 30 workers were hired 9/20 + 2/20 = 11/20 of the job is complete and 9/20 is left
So....20 + 30 = 50 workers complete the job
Every day 50 /6000 = 1/120 more of the job is now finished
So the number of days it takes to complete this is
Part that remains / Part completed each day =
(9/20 ) / ( 1/120) = 120 ( 9) /20 = 54 days
So....the total number of days = 90 + 30 + 54 = 174 days.....so....ahead of schedule