If 1800 person can finish the construction  of a building in 40 days . How many persons are needed to complete the construction of building in 24 days

Well that's really easy.

Then answer it briefly

no of days=40 ,no of people.needed=1800, we have to find first how many are needed to make the building in 1 day so for that we will divide1800?40=45 therefore 45 persons are needed to construct per day case 2 ,no of days24?no of people needed per day i.e 45 we got the answer =1080