The Attempt at a Solution
I was kind of confuse with the use of refractive index.
For the first part of question, n1 = 1.00 and n2 = 1.50. From what I know, the object is at vacuum so the n1 = 1.00.
But why for the second part of the question, n1 = 1.50 and n2 = 1.00 ? After the first refracting surface, the image should be at the vacuum too. But why we use n1 = 1.50?