# How Do You Figure Out How Many Prime Numbers Are From 1 To 100?

The Greek Mathematician Eratosthenes figured out how to solve the question how many prime numbers are between one and 100. He invented the "Sieve of Eratosthenes."

A sieve is like a strainer or something you use to filter out solid pieces that are in a liquid.

A prime number is a number that has no factors except for itself, and one. 13 is a prime number.

First you write down all the numbers from one to one hundred in order. You write the numbers in ten columns and ten rows. The first ten columns should start with one, two, three, four, five, six, seven, eight, nine, and ten.

You take out a pen or pencil. Circle 2 and strike through all of the even numbers after 2. This eliminates half of the numbers from 1 to 100. These are not prime numbers because they are multiples of 2.

Next you circle 3 and strike out all of the numbers that are multiples of 3. You count out every third number as you go across each row. These are not prime.

Next circle 5 and strike out all the multiples of 5.

Go to the next remaining number and repeat. Circle it and strike out all of its multiples.

This is the sieve of Eratosthenes.
thanked the writer.
Remember that prime numbers always ends with 1, 3, 7, 9; so there are total 25 prime numbers starting from

1, 2, 3, 5, 7
11, 31, 41, 61, 71  ---- ends with 1
13, 23, 43, 53, 73, 83 ----ends with 3
17, 37, 47, 67, 97----- ends with 7
19, 29, 59, 79, 89 ---- ends with 9

Total 25 prime number
thanked the writer. 