Eratosthenses was a Greek mathematician who figured out that to find all the prime numbers between two and some large number, you need to remove all the multiples of each number between two and your large number. Start by pressing “2” (skip over “1”), and you’ll see all the multiples of two eliminated: 2,4,6,8, etc. Next, click on “3” and so on. At some point the program will stop, and all the prime numbers between 2 and 400 will be colored red. Can you guess the biggest number you will need to click?

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