# Explanation

Method 1

Here the total work to build a floor is considered as 1 unit.

Let the Efficiency of worker of working on 1st day = x

Thus efficiency of worker working on 2nd day, 3rd day, 4th day and 5th day is 2x, 4x, 8x and 16x respectively.

From the Line graph we can see it took a total of 5 days to build 4th floor.

• x + 2x + 4x + 8x + 16x = 1
• x = 1/31

The efficiency of the worker working on second day is 2/31.

Thus the worker working on second day will take 31/2 days i.e. 15.5 days to complete the 4th floor working alone.

Increase in time = 15.5 – 5 = 10.5 days

Method 2

Let the worker working on first day complete 1 unit of work each day.

Then the worker working on 2nd day, 3rd day, 4th day, 5th day completes 2 units, 4 units, 8 units, 16 units of work each day.

From the Line graph we can see it took a total of 5 days to build 4th floor.

Thus total units of work required to complete the 4th floor = 1+2+4+8+16 = 31 units

The worker working on second day will take 31/2 days = 15.5 days.

Increase in time = 15.5 – 5 = 10.5 days