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Quick Reciprocal CalculationsIf you ever use a compass to pilot a vessel or an aircraft, chances are you'll need to calculate reciprocal headings from time to time. Now everyone knows that a heading's reciprocal can be determined by adding or subtracting 180 to or from the heading. Problem is, sometimes you don't have time to find a calculator; And adding 180 to a number in your head is a lot harder than, say, adding 200 to a number.
Enter "The Rule of Twos"
So a heading of "12" is represented as "012". "45" becomes "045". etc.
For our purposes, we'll label the digits of our numbers as follows:
The results of applying A or B in step 2 are shown in the table below.
From Steps 1 & 2 (or from the table), we see that: The Heading "Decades" 00, 18, 01, 19, 10, 28, 11, and 29 are shown in yellow in the table because they require "carrying" and "borrowing" to perform the simple addition and subtraction that is required. It might be well to memorize these 4 pairs to avoid any confusion. Just remember that we're adding 18 to the numbers in the top row to obtain the numbers in the bottom row.
Incidentally, these pairs will look familiar to anyone who pilots a plane since runways
are numbered by the first two digits of their heading. If a runway can be approached
from either direction, the two ends of the runway will be labeled with one of the pairs
of reciprocals from the table above. For example: 




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