Average speed is the total distance divided by the
a.) average distance
b.) average acceleration
c.) total time
d.) slope

C. Total time. The average speed is calculated by determining how far something has traveled and how long it took for it to get across that distance. The formula used to calculate average speed is as follows:

Average speed = Total distance/total time.

Explanation:

The wave of light is being reflected two times while going from lower to higher refractive index mediums

So there will be phase reversal two times .

Condition of destructive index

2 μ t = ( 2m+1) λ / 2

Substituting the values

2 x 1.25 x t = ( 2 x 3 + 1 ) x 600nm / 2

2.5 t = 2100 nm

t = 840 nm .