Henry Ernest Dudeney/Puzzles and Curious Problems/115 - The Eight Cards/Solution
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Puzzles and Curious Problems by Henry Ernest Dudeney: $115$
- The Eight Cards
- Rearrange these cards, moving as few as possible, so that the two columns add up alike.
- Can it be done?
- $\begin{array} {} \boxed 1 & \boxed 3 \\ \boxed 2 & \boxed 4 \\ \boxed 7 & \boxed 5 \\ \boxed 9 & \boxed 8 \\ \end{array}$
Solution
- $\begin{array} {} \boxed 1 & \boxed 3 \\ \boxed 2 & \boxed 4 \\ \boxed 7 & \boxed 5 \\ \boxed 8 & \boxed 6 \\ \end{array}$
having inverted the $\boxed 9$ to make $\boxed 6$.
Proof
As the numbers add up to $39$, it is clear this cannot be done by keeping the cards in the same orientation.
However, a veteran of these puzzles will instantly consider turning the $\boxed 9$ card round to make $\boxed 6$.
Then we see the columns add to $16$ and $20$ respectively.
So $2$ needs to be added to column $1$ and subtracted from $2$.
This can be done by exchanging the $\boxed 9$, turning it round to make $\boxed 6$, and the $\boxed 8$.
$\blacksquare$
Sources
- 1932: Henry Ernest Dudeney: Puzzles and Curious Problems ... (previous) ... (next): Solutions: $115$. -- The Eight Cards
- 1968: Henry Ernest Dudeney: 536 Puzzles & Curious Problems ... (previous) ... (next): Answers: $139$. The Eight Cards