Step 1: Write Reactants and Products.

$C_3H_8 + O_2 \rightarrow CO_2 + H_2O$

Step 2: Balance Carbon (C). 3 on Left $\rightarrow$ Need 3 on Right.

$C_3H_8 + O_2 \rightarrow \mathbf{3}CO_2 + H_2O$

Step 3: Balance Hydrogen (H). 8 on Left $\rightarrow$ Need 4 on Right (since $H_2O$ has 2 H).

$C_3H_8 + O_2 \rightarrow 3CO_2 + \mathbf{4}H_2O$

Step 4: Balance Oxygen (O). Right Side: $(3 \times 2) + (4 \times 1) = 10$. Left Side needs 10.

$C_3H_8 + \mathbf{5}O_2 \rightarrow 3CO_2 + 4H_2O$