Solve the following question given below:

Question: Looking straight downward into a rain puddle whose surface is covered with a thin film of gasoline, you notice a swirling pattern of colors caused by interference inside the gasoline film. The point directly beneath you is colored a beautiful iridescent green. You happen to remember that the index of refraction of gasoline is 1.39 and that the wavelength of green light is about 541nm.

Question I: What is the minimum possible thickness of the gasoline layer directly beneath you?

I need steps to determine the minimum possible thickness thanks!