Explanation:

- We first calculate the total man-hours needed for the initial task: 15 men × 15 days × 8 hours/day = 1800 man-hours.

- This is the work required to complete one unit of work.

- The new task is twice the work, so it requires 2 × 1800 = 3600 man-hours.

- N men are to complete this task in 20 days working 10 hours a day. This gives us N × 20 days × 10 hours/day = 200N man-hours.

- We set up the equation: 200N = 3600.

- Solving for N: N = 3600 / 200 = 18.

- Correct option: Option 3 - 18

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