Pipe P can fill a tank in 4 hours while Q can fill it in 6 hours. Working separately pipe R can empty the whole tank in 4 hours. He opened the pipe P and Q simultaneously to fill the empty tank, he wanted to adjust his alarm so that he could open the pipe R, when it was half filled, but he mistakenly adjusted his alarm at time when his tank would be 3/4th part filled. What is time difference between in both cases to fill the tank fully ?