The scheduled arrival time should have been 8:00pm with a flight time of 8 hours.

8 hours = 32 periods of 15 minutes.

Over 32 periods the plane lost 16 minutes

Over 1 period the plane lost

The plane fell behind by 30 seconds every 15 minutes.

8 hours = 32 periods of 15 minutes.

Over 32 periods the plane lost 16 minutes

Over 1 period the plane lost

^{16}/_{32}minutes = 30 seconds.The plane fell behind by 30 seconds every 15 minutes.