Question

What is the longitude of a place where the local time is 1:00p.m. , when it is 4:00a.m. at Chicago

Answer 1

As the earth rotates from west to east, those places that lie east of Greenwich are ahead than the places that lie to the west of Greenwich.

When it is 4 AM at Chicago (set to 90°W). GMT is (6hrs ahead) 10 AM. 
From there 1:15 PM is 3¼ hrs ahead (to east), 
3¼ X 15°E 
= 48° 45'E.
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