P, Q and R together can make a table in 40 minutes. P and Q together can make it in 60 minutes. How much time (in minutes) will R alone take to make the table?
X, Y and Z can do a piece of work in 30,40 and 50 days respectively. All three of them began the work together but Y left 4 days before completion of the work. In how many days was the work completed?
A and B together do a job in 12 days and A could do the job in 20 days if he worked alone. How many days would B take to do the job if he worked alone?