Motion in a Straight Line

Two trains 'A' and 'B' of length ' l ' and ' 4l ' are travelling into a tunnel of length ' L ' in parallel tracks from opposite directions with velocities 108‌km∕h and 72‌km∕h, respectively. If train 'A' takes 35 s less time than train 'B' to cross the tunnel then, length ' L ' of tunnel is:
(Given L=60l )