Answer : 1. "50 days"
Explanation :

Answer: A) 50 days Explanation: 50 men can build a tank in 40 days  Assume 1 man does 1 unit of work in 1 day Then the total work is 50×40 = 2000 units   50 men work in the first 10 days and completes 50×10 = 500 units of work 45 men work in the next 10 days and completes 45×10 = 450 units of work 40 men work in the next 10 days and completes 40×10 = 400 units of work 35 men work in the next 10 days and completes 35×10 = 350 units of work So far 500 + 450 + 400 + 350 = 1700 units of work is completed and   Remaining work is 2000 - 1700 = 300 units   30 men work in the next 10 days. In each day, they does 30 units of work.   Therefore, additional days required = 300/30 =10   Thus, total 10+10+10+10+10 = 50 days required.