Apparent depth with multiple indices of refraction

[KBriggs]

Homework Statement

A penny is located at the bottom of a barrel of water 1m deep. There is a 20cm thick layer of oil on top of the water. To an observer at normal incidence, what is the apparent depth of the penny. n for water is 1.33, for oil it is 1.5

Homework Equations

Snell's law, lots of ray diagrams

The Attempt at a Solution

I have already shown that to an observer looking through just the water, the apparent depth is 0.75m. I am just not sure how to generalize this for the other layer. Can someone point me in the right direction?

 

[willem2]

The method is the same, only you have to use Snell's law twice, at the water oil, and oil/air boundaries.