How long (in ns) does it take light incident perpendicular to the glass to pass through this 8.9cm -thick sandwich?

A 5.2cm -thick layer of oil (n=1.46) is sandwiched between a 1.4cm -thick sheet of glass and a 2.3cm -thick sheet of polystyrene plastic (n=1.59).

There are 2 steps to solve this one.