When 55g C3H8 combusts with 30L of oxygen, how many total atoms can be produced?

This equation is balanced by inspection. C3H8 is burned in an excess of oxygen. You will get CO2. Since C3H8 has 3 carbons, start by getting 3CO2 molecules on the product side.

C3H8 + O2 - - > 3CO2

Note than you need 6 oxygen atoms so start with 3O2

C3H8 + 3O2 - - > 3CO2

What happens to the Hydrogens? In a combustion reaction in oxygen the hydrogen is converted to H2O. Since you have 8H you will need 4 H2O on the product side.

C3H8 + 3O2 - - > 3CO2 + 4H2O

Inspection reveals that the C and H are balanced but that we need 4 more O atoms on the reactant side

C3H8 + 5O2 - - > 3 CO2 + 4H2O

it is now balanced: 3 carbons, 10 oxygen's and 8 hydrogens on both sides of the reaction.