Mathematics · Time and Work

Three pipes P, Q and R can fill an empty water tank in 24, 36 and 48 minutes, respectively. A pipe X can empty the full tank in 1 hour. The three pipes P, Q and R are opened at the same time and after 4 minutes R is closed and X is opened. After 6 more minutes P is closed. If the remaining tank is filled when only pipes Q and X are open, then find the time taken by Q and X to fill that part of the tank.