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 ?

Answer: Option B

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
$$\begin{array}{rl}& =\frac{72}{6}\\ & \text{ = 12 days}\\ & \text{Hence,}\\ & \text{Total time}=12+6=18\text{ days}\\ & \text{Extra time}=18-12=6\text{ days}\end{array}$$