Answer :
I'll assume you are referring to Manila, Philippines. This city is given the time zone GMT+8 meaning that you are 8 hours ahead of Greenwich Mean Time (GMT).
Anacortes, Washington (in the United States) is meanwhile assigned the time zone code of GMT-7. Normally it is GMT-8, but we add 1 hour due to Daylight Savings Time (DST). The negative indicates your sister is 7 hours behind GMT.
Draw out a number line. Mark -7 and 8 on the number line. Then notice there are 15 spaces between the values. Or you can subtract and use absolute value.
|p-q| = |-7-8| = 15 where p = -7 is the GMT value for Anacortes and q = 8 is the GMT value for Manila.
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So all of that previous paragraph boils down to this: there's a 15 hour gap between you and your sister.
Notice how 15 hours is 3 hours longer than 12 hours because 15-12 = 3.
This means that to quickly find your sister's time, you will subtract 3 from your local time and swap AM to PM, or vice versa.
Examples:
- It is currently 8:00 PM in Manila. Therefore it is 5:00 AM in Anacortes because 8-3 = 5, and the PM swapped to AM.
- Another example: It is currently 6:00 AM in Manila. Subtract off 3 and swap AM to PM to determine it is 3:00 PM in Anacortes.
Reverse this process to determine what time it is in Manila if you know the time in Anacortes.
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