A plane left on time at noon on Monday. It arrived at its destination at 8:16PM the same day, which was 16 minutes late. On average, how many seconds did the plane fall behind every fifteen minutes?

The plane arrived on 8:16 pm and was 16 minutes late. This means the arrival time of plan was 8:00 pm.

The plane left on noon i. e. 12:00 pm.

So total expected time of the journey was = 8 hours

8 hours means 8 x 60 minutes = 480 minutes

480 minutes can also be seen as 32 groups of 15 minutes each as 15 x 32 = 480.

So, in 32 groups of 15 minutes each the plane was late by a total of 16 minutes.

So, in every 15 minute the plane was late by 16/32 minute = 0.5 minute

Thus, we can conclude that on average the plane fall behind 0.5 minutes in every 15 minutes