Here's how to calculate the number of atoms in the molecule C7H8N4O2:

* Count the subscripts:

* Carbon (C): 7 atoms

* Hydrogen (H): 8 atoms

* Nitrogen (N): 4 atoms

* Oxygen (O): 2 atoms

* Add the atoms together: 7 + 8 + 4 + 2 = 21 atoms

Therefore, there are 21 atoms in the molecule C7H8N4O2.