Sita can finish typing a 100 page document in 9 hours, Mita in 6 hours and Rita in 12 hours. How long will they take to type a 100 page document if they work together?
Work done by Sita in 1 hour = 
Work done by Mita in 1 hour = 
Work done by Rita in 1 hour = 
Work done by them together in 1 hour = 
∴ Time taken by them to complete the work together = 
5