John stpes on an escalator to leave a subway. Then, 4 seconds later, Jane steps on a parallel escalator to enter the subway. They pass 12 seconds...

John stpes on an escalator to leave a subway. Then, 4 seconds later, Jane steps on a parallel escalator to enter the subway. They pass 12 seconds after that. If both escalators are travelling at a speed of 5 feet per second, what is the length, l, of the escalators