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?
1. A and B together do a job in 15 days and A alone could do the same job in 20 days. How many days would B take to do half the job if he worked alone?