SOLUTION: Find the distance between points X(30N, 40W) and Y(50N, 140E) on the same great circle.

Notice, that since 40 + 140 = 180 degrees, it means that these two points lie on the same great circle 

(section of the Earth globe), which goes through the North and the South geographic poles,

and both the points are in the Northern hemi-sphere.



Therefore, the distance between them on the Earth surface is  

     =  = 2234 kilometers,


where R = 6400 km is the Earth radius (commonly used value for such calculations), 

and 20 represents the central angle, or arc angle between the given points 
(difference of their altitudes, in geographic context).