Some books write photosynthesis as 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂…

Step 1. Left side. $6 \times 2 + 6 \times 1 = 12 + 6 = 18$ oxygen atoms.

Step 2. Right side. Glucose has $6$, and $6\text{O}_2$ has $12$: $6 + 12 = 18$ oxygen atoms. Carbon ($6$ and $6$) and hydrogen ($12$ and $12$) also match, so the short equation is balanced.

Step 3. Why 12 waters. All the oxygen given out comes from water. Six $\text{O}_2$ molecules need $12$ oxygen atoms, so $12$ water molecules must be split. Some new water is made again, so $6$ appear on the right. The longer equation shows where the oxygen really comes from.