Some carpenters promised to do a job in 9 days but 5 of them were absent and remaining men did the job in 12 day. The original number of carpenters was ?

Let there were N carpenters in the beginning
According to the question,
$$\frac{{{{\text{N}}_{{\text{men}}}} \times {9_{{\text{days}}}}}}{{{{\text{1}}_{{\text{work}}}}}} = $$     $$\frac{{{{\left( {{\text{N}} - 5} \right)}_{{\text{men}}}} \times {{12}_{{\text{days}}}}}}{{{{\text{1}}_{{\text{work}}}}}}$$
$$\eqalign{ & {\text{3N}} = {\text{4N}} - 20 \cr & {\text{N}} = {\text{20 men}} \cr} $$