Let a man can do 1 unit/day ⇒ Total work = 50×28=1400 man days So, work done in first 10 days by 50 men = 10×50=500 man days Then, work done in next 10 days by 40 men = 10×40=400 man days Similarly, 300 man days after next 10 days and 200 after that. Thus, 500+400+300+200=1400 man days ⇒ Total days required = 40 days