A man is allocated a task. He doubles the task done everyday. If the man completely does the task in 18 days, how many days did it take for the man to complete 25% of the task?
100% of task = 18 days
As he doubles the task everyday. So,
50% of task = 17 days
25% of task = 16 days.
Alternative Solution :
Let the amount of work the man did on first day be x and total work to be done be S.
As the amount of work he did on next day would be 2 times of it’s previous day, we can write amount of work he did on 2nd day as ‘2x’, 3rd day ‘4x’, 4th day ‘8x’ and so on….
We can write it as a GP as follows :
S = x+2x+4x+8x+….. (till 18th day)
S = x*(218-1)/(2-1) [Formula of summation of GP S=a(rn-1)/(r-1)]
S=x*(218-1) – equation (1)
According to ques we have to find the day when he would complete 25% of the total work, then ‘S’ becomes ‘S/4’
We can write the expression as
S/4 = x+2x+4x+8x+…. (till nth day)
Substituting value of S from equation (1)
We can neglect 3 in comparison of 218
So answer is 16 days.