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Mathematics
Quick fact
The divisibility rule for 13 works because multiplying the last digit by 4 and adding it to the rest of the number is equivalent to checking the original number modulo 13, due to the fact that 10 is congruent to -3 modulo 13, and 4 is the modular inverse of 10.
Why this is interesting
You can test if a number is divisible by 13 with a simple trick—but why does it work, and what hidden patterns make such shortcuts reliable?
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Subjects · study & training · careers
Choose something you are considering. FACTREE will give you a clear picture of what it contains, how its main parts connect and where it could lead.
This private catalogue connects 31 subjects, study routes and careers across six broad areas. Public Maps and learner routes remain off.
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Structuring the Programming Map…
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