Twelve men are assigned a job to be finished in 30 days. After 10 days it is observed that only one-sixth of the job is completed. How many extra men are required to complete the rest of the job within the scheduled time?

30 men are needed in all, so 18 more must join.

12 men in 10 days did 1/6, i.e. 12 x 10 = 120 man-days for 1/6.

Whole job = 120 x 6 = 720 man-days.

Remaining 5/6 = 600 man-days, to be done in 20 days.

Men required = 600 / 20 = 30, extra = 30 - 12 = 18.