I am not sure if the above method will be accepted, but what I have done is to compare the time saved when Nixon ran 16 steps instead of running 9 steps. Since the total number of steps is the same, thus we can calculate the total time saved when Nixon decides to walk the entire flight of stairs without running at all (either for the 9 or 16 steps answer will be the same when you add it to the corresponding time). You can read my explanation and thought process for better understanding. You can try to use a model to represent this but not sure how it will work.