ANSWERS: 4
  • How to find ALL prime numbers between 20 and 50. Write down all odd numbers between 20 and 50: (those that are not divisble by two, the first prime): 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 Cross out those divisible by three (the second prime): 23 25 29 31 35 37 41 43 47 49 Cross out those divisible by five (the third prime): 23 29 31 37 41 43 47 49 Cross out those divisible by seven (the fourth prime): 23 29 31 37 41 43 47 Now I can stop, because the remaining numbers are less than seven squared. This is called The Sieve of Eratosthenes.

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