4 men can check a certain number of exam papers in 8 days, working 5 hours per…

Concept: Total work done on a task is measured in man-hours: Total Work = (Number of Men) x (Number of Days) x (Hours worked per day). For a fixed job, this product stays constant across any team/schedule that completes it -- so when the workload itself changes (here it doubles), the new team's men x days x hours must equal the new, doubled total, letting the two scenarios be equated directly.

Application:

1. Work done on the original task = 4 men x 8 days x 5 hours/day = 160 man-hours.

2. The new task is double the number of papers, so the work required = 2 x 160 = 320 man-hours.

3. Let h = hours worked per day in the new scenario. With 2 men working for 20 days, work done = 2 x 20 x h = 40h man-hours.

4. Equate work done to work required: 40h = 320.

5. Solve for h: h = 320 / 40 = 8 hours per day.

Cross-check: Check: 2 men x 20 days x 8 hours/day = 320 man-hours, exactly double the original 160 man-hours -- consistent with the doubled workload.