Question
If it's 3pm in the following cities, what time is it in Bangkok, Thailand?
London, New York, Chicago, Vancouver (Canada), Paris, Berlin, Moscow, Dehli, Beijing, Melbourne, Brisbane, Cape Town
Answer
London = GMT +1 => +6 hours => 15 + 6 = 21:00 => 9pm
New York = GMT -5 => +14 hours 15 + 14 = 29:00 => 5am the next day
Chicago = GMT -6 => +15 hours 15 + 15 = 30:00 => 6am the next day
Vancouver (Canada) = GMT -8 => +17 hours 15 + 17 = 32:00 => 8am the next day
Paris = GMT +2 => +5 hours 15 + 5 = 20:00 => 8pm
Berlin = GMT +2 same answer as Paris
Moscow = GMT +3 => +4 hours 15 + 4 = 19:00 => 7pm
Dehli = GMT +5:30 => +1.5 hours 15 + 1.5 = 16.5 => 4:30pm
Beijing = GMT +8 => -1 hours 15 -1 = 14:00 => 2pm
Melbourne = GMT +10 => -3 hours 15 - 3 = 12:00 => noon
Tokyo = GMT +9 => -2 hours 15 - 2 = 13:00 => 1pm
Cape Town = GMT +2 same answer as Paris
Analysis
The globe is divided into 24 timezones, each one labeled with a different time adjustment, plus or minus, from GST, or Greenwich Standard Time.
Here's a map showing the different timezones (courtesy of gifex.com):
The different colours, running vertically, represent areas within the same timezone (so New York and Peru are in the same time zone).
It's hard to see details on this map, so I'll use a different online tool to get the timezone adjustments:
Bangkok = GMT +7
London = GMT +1
New York = GMT -5
Chicago = GMT -6
Vancouver (Canada) = GMT -8
Paris = GMT +2
Berlin = GMT +2
Moscow = GMT +3
Dehli = GMT +5:30
Beijing = GMT +8
Melbourne = GMT +10
Tokyo = GMT +9
Cape Town = GMT +2
When doing timezone problems, something that can help greatly is using a 24-hour clock, and so 3pm is the same as 15:00.
Let's do London first. When it's 15:00 in London, it's 6 hours later in Bangkok (GMT +1 vs GMT +7). And so 15:00 in London is 21:00 in Bangkok (or 9pm).
How about with New York? New York is at GMT -5, and so the difference is 14 hours (it takes 5 hours to get to GMT and then another 7 to Bangkok). And so 15:00 in New York is 29:00 in Bangkok. But there are only 24 hours in a day! So we subtract 24 but add one day, to get 5:00 (or 5am) the next day.
Let's do this for all the cities:
London = GMT +1 => +6 hours => 15 + 6 = 21:00 => 9pm
New York = GMT -5 => +14 hours 15 + 14 = 29:00 => 5am the next day
Chicago = GMT -6 => +15 hours 15 + 15 = 30:00 => 6am the next day
Vancouver (Canada) = GMT -8 => +17 hours 15 + 17 = 32:00 => 8am the next day
Paris = GMT +2 => +5 hours 15 + 5 = 20:00 => 8pm
Berlin = GMT +2 same answer as Paris
Moscow = GMT +3 => +4 hours 15 + 4 = 19:00 => 7pm
Dehli = GMT +5:30 => +1.5 hours 15 + 1.5 = 16.5 => 4:30pm
Beijing = GMT +8 => -1 hours 15 -1 = 14:00 => 2pm
Melbourne = GMT +10 => -3 hours 15 - 3 = 12:00 => noon
Tokyo = GMT +9 => -2 hours 15 - 2 = 13:00 => 1pm
Cape Town = GMT +2 same answer as Paris
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