a man starts a piece of work. starting from the second day onward a new man joins.with every new man joining the work that each man can do per day doubles. the work is completed in 5 days. on which day would they have completed the work if the work that each of them could do per day had remained constant?

16th day.
Let efficiency of men be 1.
So on first day work done is 1.
As efficiency doubles with ever passing day and one man adds up, so on 2nd day it’s ( 2 x2 =4)
3rd day= (3men x 4 = 12)
4th day = ( 4men x 8 =32 )
5th day = ( 5 men x 16 = 80)
So total work done in 5 days is ( 1+4+12+32+80) = 129.
Now as efficiency of men remains constant now, only the no. Of men increases by 1.
So (1+2+3+4+5……….) is the amount of work done on consecutive days.
Sum~ n/2(n+1)=129, so on 16th day work is completed.