Calculating Light's Travel Time Through Layered Media

[hsphysics2]

Homework Statement

A 5.9cm-thick layer of oil (n=1.46) is sandwiched between a 1.3cm -thick sheet of glass and a 2.4cm-thick sheet of polystyrene plastic (n=1.59)

How long (in ns) does it take light incident perpendicular to the glass to pass through this 9.6cm -thick sandwich?

Homework Equations

n=c/v

v=fλ

The Attempt at a Solution

I am very confused to this problem because I am unsure of how to incorporate the length of the different mediums into the equations.

 

[Goddar]

Hi.
You only need your first equation to give you v in each medium, as a function of c. Now given the value of the speed of light (which you can express in cm/s), the question is similar to:
if i walk through a A-meters wide park at B meters/s then across a C-meters wide boulevard at D meters/s, how long did it take me?..