Question

Assuming that the jet cruises at this constant speed, how long do the pilots' watches measure for a complete circumnavigation of the globe? the radius of the earth is about 6380 km.

Answer 1

Speed = distance/time
Constant speed means no acceleration so this formula can be used
The question says THIS constant speed so I’m assuming in an earlier part of the question you are told the speed at which it cruises.
Therefore,
Using the speed given
Speed divided by 6380km =time