Ram can complete a piece work in 15 days, Rohan in 25 days, and Rohit in 30 days. Rohan and Rohit worked together for 2 days and then Rohit was replaced by Ram. In how many days altogether was the work completed?

P, Q and R can complete a piece of work in 9, 12 and 18 days, respectively. Working together, how much work can they complete in one day?

Ram and Ramesh can do a work in 12 days, Ramesh and Somesh in 15 days and Somesh and Ram in 10 days. If Ram, Ramesh and Somesh work together, in how many days will they complete the work?

A, B and C can do a piece of work in 20, 30 and 60 days, respectively. In how many days can A do the work if he is assisted by both B and C on every third day?