X, Y and Z alone can do a certain piece of work in 24 days, 36 days and 48 days respectively. All of them started the work together, but after ‘k’ days X left the job. Y and Z continued the work, but after another ‘k’ days Y also left. Now, the remaining work is completed by Z alone with 33.33% increased efficiency in 6 days. What is the value of ‘k’?