Renata walks down an escalator that moves up and counts $150$ steps. Her sister Fernanda climbs the same escalator and counts $75$ steps. If the speed of Renata (in steps per time unit) is three times the speed of Fernanda, determine how many steps are visible on the escalator at any time.
The answer is 120 steps.
This is my first post ever on stackexchange, I hope I've done nothing wrong. Also, English isn't my first language, so I'm sorry if there's any spelling mistakes i my problem, I translated it the best I could. I've tried to solve it many times, but the number of equations doesn't match the number of unknown values.