Explanation:

- Initially, a certain number of men can finish the work in 6k days.

- To finish the work in less time, they must increase their workforce.

- Formula: Work = Number of Men × Days

- Original Situation: Men × 6k = Work

- New Situation: (Men + Increase) × 5k = Work

- Setting the equations equal, you get: (Men × 6k) = (Men + Increase) × 5k

- Simplifying, you find: Increase = Men × (1/5)

- This means the workforce needs to be increased by 1/5, which equals 20%.

Option 3: 20% is correct.