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