(related to Problem: The Four Sevens)

The way to write four sevens with simple arithmetical signs so that they represent $100$ is as follows:— $$\frac{7}{.7}\times\frac{7}{.7} = 100.$$

Of course, the fraction, $7$ over decimal $7,$ equals $7$ divided by $\frac{7}{10}$, which is the same as $70$ divided by $7,$ or $10.$ Then $10$ multiplied by $10$ is $100,$ and there you are! It will be seen that this solution applies equally to any number whatever that you may substitute for $7.$

Thank you to the contributors under CC BY-SA 4.0!



Project Gutenberg

  1. Dudeney, H. E.: "Amusements in Mathematics", The Authors' Club, 1917

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