5
$\begingroup$

This puzzle is due to the amazing Martin Gardner and I found it here.

Can you find which digit each letter represents to make the following sum work? Each letter is a single unique digit between 0 and 9.

..SEVEN
+SEVEN
+SEVEN
+SEVEN
+SEVEN
+SEVEN
+SEVEN
................
FORTY9

$\endgroup$
8
$\begingroup$

Obviously, N must be

seven, because 7 is the only number that gives a product ending in 9 when multiplied by 7.

Then, let's take a guess to get acquainted with the types of trouble we are going to face. We get the simplest calculations if we set E=0, so let's try that.

Looks like we'll then need three multiplications (x * 7 = yz) with all the digits unique. That seems easy:

$$ 7 \times 7 = 49 $$ $$ 8 \times 7 = 56 $$ $$ 3 \times 7 = 21 $$

It seems we have accidentally stumbled on a solution. Even more than that, since we have zeroes in all the strategically important spots, the order in which we place the multiplications doesn't matter. This means there are at least two solutions:

$$ 80307 \times 7 = 562149 $$ $$ 30807 \times 7 = 215649 $$

A double solution doesn't really seem Gardner's style though. I'll try and see if I can't find the original..

EDIT: Found it. The original has the extra stipulation that "SEVEN" must be divisible by 7, which rules out the bigger answer. Looks like SciAm forgot to include that extra condition in their 2014 reprint of the puzzle.

enter image description here

Source: "Index to Mathematical Problems 1975-1979, edited by Stanley Rabinowitz, Mark Bowron", highlighting mine.

| improve this answer | |
$\endgroup$
  • $\begingroup$ You got it! Well done. Official answer is here: scientificamerican.com/article/… $\endgroup$ – Dmitry Kamenetsky May 11 at 7:40
  • 2
    $\begingroup$ Why would we want an "official" answer though? I thought I just showed beyond any reasonable doubt that the question is misstated in the 2014 article you copied it from, and the magazine's answer only answers the question as it was originally stated in the seventies. $\endgroup$ – Bass May 11 at 8:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.