Is This Prime?

In the past few days, I, my friends, and a lot of Twitter people have been trying to beat each other’s scores in the game “Is This Prime?”.

The game itself is simple; you are shown random integers on the screen, and you need to guess whether they are prime or composite. It is fast-paced and fun, and a good opportunity to exercise some mental math. Below we have listed a few tips which can help you beat our personal record of 67 points.

  1. Memorize as many numbers as possible. Knowing the multiplication table up to 10×10, it should be easy to learn by heart whether each number up to 100 is prime or composite.
  2. Pay attention to the last digit. If it is 5, then the number is composite (unless it is =5).
  3. Check whether the sum of the digits is divisible by 3. If it is, then the number is composite (unless it is equal to 3).
  4. If the number is between 100 and 300, check whether the sum of the first and the third digits equals the second digit. If this is true, then the number is divisible by 11, and therefore it is composite. 209 is the only other number in this range divisible by 11.
  5. Remember the sneaky composites: 119, 133, 161, 169, 247, 253, 259, 289, 299.
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Puzzle Newsletter (Post) (#10)
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