Work is always taken as one unit. Construction of a building, filling water in the tank, painting a room etc.
To complete a job, a man will do the same amount of work on each day of the total number of days he takes to complete that job.
If a man can do a piece of work in 8 days, then his one day’s work is .
If a man’s one day’s work is then he can complete the total work in 4 days.
If a man can complete a piece of work in A days and another man can complete the same work in B days,
then they together can complete the work in 
days.
Similarly, three persons A, B and C together can complete in 
days.
Ex: A can complete a piece of work in 6 days and B in 8 days. In what time they complete if they work together?
MAN DAYS:
If 6 men can complete a work in 4 days, then the number of man days required to complete that work is 6 × 4 = 24.
Whatever may be the number of persons working on that, the total number of man days required for that work will be 24.
Ex:18 men together can complete a work in 14 days. In how many days 12 men finish that work?
A) 5 dyas B) 6 days C) 10 days D) 8 days E) None of these
Men and Time are inversely proportional i.e., when more men work, they take less time to complete the work. Similarly when less men work, they take more time to complete the work.
Men and Work are directly proportional i.e., when more men are there, they do more work and less men are there, they do less work.
Similarly, Time and Work are also directly proportional. If men work for more time then they do more work and less time then the work is also less.
The relation among these variables can be shown in a formula
If M is Men, D is Days (Time) and W is work then 
Ex: A contractor employs 12 men to complete a work in 15 days. But after 8days he notices that only 30% work has been completed. In order to finish the work in the given time, how many more men he has to recruit?
A) 32 B) 24 C) 20 D) 9 E) None of these
To complete the remaining work 32 men are required
Additional men required 32 - 12 = 20
Hence answer is (c)
