There is a 10-hour
time difference between San Francisco and Cairo,
Egypt. If Helix called at 3:00
PM from California,
it would be 1:00 AM the next day in Eqypt.
His grandfather might be asleep.
Global time is calculated from an imaginary
line, called the Prime Meridian
that extends from the North Pole to the South Pole through
Each time zone to the east
of Greenwich is numbered with a negative (-) number;
while each zone to the west is
numbered with a positive (+) number.
Cairo is at 2,
which indicates it is two time zones east of Greenwich.
San Francisco is at +8,
which indicates it is 8 time zones west of Greenwich.
This means that a total of 10
time zones separates the two cities. Therefore,
3:00 PM in San Francisco is 1:00 AM the next morning in
Another way to do this challenge follows:
San Francisco is at +8 on the map. Cairo is at 2.
Using mathematical operations,
+ 8 (2) = 10. When it is 3:00
PM in San Francisco, it is 10 hours later in Cairo, or
1:00 AM the next morning.