A job can be complete by 12 men in 12 days. How many extra days will be needed to complete the job if 6 men leave after working for 6 days ?

According to the question,
Total work
= 12 M × 12 D
= 144 units
Work done by 12 men in 6 days
= 12 × 6
= 72 units
Rest work
= 144 - 72
= 72 units
Required time for 6 men to complete the work
$$\eqalign{ & = \frac{{72}}{6} \cr & {\text{ = 12 days}} \cr & {\text{Hence,}} \cr & {\text{Total time}} = 12 + 6 = 18{\text{ days}} \cr & {\text{Extra time}} = 18 - 12 = 6{\text{ days}} \cr} $$