Question

Give a reason for the following :

When it is noon at Cairo (30°E), the local time in New York (75°W) is 5.00 a.m.

Answer 1

New York	Greenwich	Cairo
75° W	0°	30°E
	?	12.00

Longitude of Cairo = 30° E
Longitude of New York = 75° W
Difference in Longitude = 30° + 75° = 105°
(Add because Cairo is in the East of Greenwich and New York is in the West of Greenwich).
∴ Difference in time = 105 × 4 = 420 minutes = 7 hours
As New York is situated in the West of Greenwich, its local time will be behind that of Cairo. So we subtract.

	Hours	Minutes
Local time at Cairo =	12	0
Subtract =	7	00
Local time at New York =	5	00 (a.m.)