2 steps is done in 1 second.
80 steps will be done in ? seconds (Let this answer be a)
In 'a' seconds, Dan took 80 steps down.
In 'a' seconds, the escalator also got 1.5a steps less from where Dan started.
So, when Dan started, there were 80 + 1.5a steps in all.
Now, the escalator produces 1.5 new steps each second.
There is initially 80 + 1.5a steps to climb, but after some time, there are more steps to climb, and how much more, is given by 1.5b (where b is the new time Dan takes)
Total number of steps = 80 + 1.5a + 1.5b = 200
Solve for b, the time.
200 steps is done in b seconds
Speed = 200/b steps/second.
d) Can you give this one a try now?