Lithium is Li
Oxygen gas is O2
Lithium oxide is Li2O
Unbalanced Equation is Li + O2 --> Li2O
There's one O on right side, 2 Os on left, so make coefficient 2 on right side to balance the oxygens.
Li + O2 --> 2 Li2O
Now balance the lithiums. There are 4 Lis on right side; 1 Li on left, so make Li coefficient 4 to balance the lithiums.
4 Li + O2 --> 2 Li2O
This equation is now fully balanced.
Therefore balanced equation is:
4 Li + O2 --> 2 Li2O