Explanation:

- Let's say the original number of carpenters was \( x \).

- They promised to complete the job in 15 days, meaning they would do 1/15 of the job in one day.

- If 3 carpenters were absent, then \( x - 3 \) carpenters completed the job in 20 days, doing 1/20 of the job in one day.

- We set up the equation: \( \frac{x-3}{x} = \frac{15}{20} \), which simplifies to \( \frac{3}{4} \).

- Solve \(\frac{x-3}{x} = \frac{3}{4}\) to find \( x = 12 \).

?? Option 3: 12 is the correct answer.