How many atoms make up 20.0 grams of chalk (caco3)?

First, find the molar mass of CaCO3 (100.0869 grams, but you can usually look on a periodic table for a less precise number and still be fine).

Convert grams to moles, and moles to atoms

20.0 grams * = 1.20 * .

Check if the answer makes sense.

If you have 20 grams of chalk, you have ~20% of a mole of chalk.  This means you have ~20% of the atoms a normal mole of chalk would have.

1.2 / 6.02 = 0.199, which is very close to 0.2 a.k.a. 20%