How do you find out how many prime number from 1 to 10000?

Answer 1

Start counting from 2. For each number, if it has factors besides 1 and itself,

then skip it; but if it doesn't have, then write it down. When you reach 10,000,

you'll have the list, and you can count them.

Here's another way to do it, which is probably easier:

-- Write down all the numbers from 2 to 10,000.

-- Start with 2. Cross all multiples of 2 off the list.

-- Then go to 3. Cross all multiples of 3 off the list.

-- 4 is gone ... it was a multiple of 2 ... so go on to 5. Cross all multiples of 5 off the list.

-- 6 is gone ... it was a multiple of 2 and also of 3 ... so go on to 7. Cross all multiples

of 7 off the list.

-- 8, 9, and 10 are gone. Go on to 11. Cross all multiples of 11 off the list.

-- Keep going like this, until you reach 5,000 . You'll have the list, and you can count them.