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The inheritance of the 17 camels (Arab tradition)

Numerical territoryLevel 2 · Core · ●●○○○

A father leaves 17 camels to his three sons with these conditions: the oldest gets $\frac{1}{2}$, the second gets $\frac{1}{3}$ and the youngest gets $\frac{1}{9}$. Splitting camels is not allowed. How can the distribution be made by exactly complying with the will?
Classic recreational arithmetic puzzle in the Middle East

Hints

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  1. There is no magic: it is a trick of least common multiple and addition of fractions. The extra camel only allows you to execute an entire distribution compatible with the will.
  2. That is why it is convenient to take 17 to 18: it makes all partitions whole.
  3. A camel is temporarily added to work on 18 and then removed.

Solution

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Answer: A camel is temporarily added to work on 18 and then removed.
1) Arithmetic idea
The fractions of the will are

$$ \frac12,\ \frac13,\ \frac19, $$

and its sum is

$$ \frac12+\frac13+\frac19=\frac{17}{18}. $$

That is why it is convenient to take 17 to 18: it makes all partitions whole.
2) Distribution over 18

  • to the greatest: $18/2=9$,
  • to the second: $18/3=6$,
  • to the minor: $18/9=2$.

Total delivered:

$$ 9+6+2=17. $$

Exactly 1 camel left: the one temporarily added.
3) Methodological conclusion
There is no magic: it is a trick of least common multiple and addition of fractions. The extra camel only allows you to execute an entire distribution compatible with the will.


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