A shopkeeper has a job to print certain number of documents and there are three machines P, Q and R for this job. P can complete the job in 3 days, Q can complete the job in 4 days and R can complete the job in 6 days. How many days the shopkeeper will it take to complete the job if all the machines are used simultaneously ?

Let the total number of documents to be printed be 12.

 The number of documents printed by P in 1 day = 4.

 The number of documents printed by Q in 1 day = 3.

 The number of documents printed by R in 1 day = 2.

 
Thus, the total number of documents that can be printed by all the machines working simultaneously in a single day = 9.

Therefore, the number of days taken to complete the whole work = 12/9 = 4/3 days.