For the second part you are correct, but you seem unsure of your answer. I will explain.
If three workers acan do this job together in 18 hrs, they they each take 54 hrs to do it alone.
So the answer to the first part is 4/54 = 1/x = 54/4 = 13.5.
And the second is just 2/54 = 1/x = 54/2 = 27.
Good joob Rich, you had the intuition - next will come the confidence!!