A match has about 21 milligrams of red phosphorus coating the tip. How many atoms of phosphorus is this?

4.083 * 10^20 atoms.

Explanation:

One Mole of phosphorus contains 6.022 * 10^23 atoms (Avogadros number) '

Since 1 mole of Phosphorus has a mass of 30.974 grams, 21 milligrams has

6.022 * 10^23 * 0.021 / 30.974

= 0.004083 * 10^23

= 4.083 * 10^20