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.
