Albert goes upstair on a moving floor. When he lets the moving floor make him go upstairs, it takes him 30 seconds. When he walks while being on the moving floor, it takes him 10 seconds. How much time would it take him to go upstair if the moving floor didn't work anymore (So, you can only walk) I have zero ideas what to do in this case. I think it might have something to do with relative speeds, but I'm not sure.