Scaling examiners’ effort: 9 examiners can examine a certain number of answer books in 12 days by working 5 h/day. How many hours/day should 4 examiners work to examine twice that number in 30 days?

Introduction / Context:
This is a proportional scaling of man-hours with a changed team size and target workload (twice the original). We equate required man-hours using the same per hour productivity and solve for the daily hours needed in the new scenario.

Given Data / Assumptions:

• Original: 9 examiners, 12 days, 5 h/day ⇒ base workload W
• New: 4 examiners, 30 days, H h/day ⇒ target workload 2W
• Efficiency per examiner-hour is constant

Concept / Approach:
Man-hours scale linearly with workload. Therefore, 9*12*5 corresponds to W; for 2W, we require twice that many examiner-hours distributed as 4*30*H. Solve for H.

Step-by-Step Solution:

Verification / Alternative check:
Back-check: 4 * 30 * 9 = 1080 man-hours, exactly double the original 540, so twice the workload is feasible in 30 days.

Why Other Options Are Wrong:

• 11, 13, 17, 7: These values do not yield 1080 man-hours with 4 examiners over 30 days.

Common Pitfalls:
Forgetting to double the original workload; or equating 9*12*5 directly to 4*30*H without the factor of 2, which would understate hours/day.

Final Answer:
9