1 quart is approximately 9.464 x 10⁻⁴ cubic meter, so the amount of oil that was spilled is about
0.50 qt = (0.50 qt) * (0.0009464 m³/qt) = 4.732 x 10⁻⁴ m³
The pool has a cross sectional area of (25.0 m) * (30.0 m) = 750 m² = 7.5 x 10² m².
Assuming the oil spreads completely across the surface of the pool, you can get its thickness by dividing the volume of oil by this cross sectional area.
(4.732 x 10⁻⁴ m³) / (7.5 x 10² m²) = 4.732/7.5 x 10⁻⁶ m ≈ 6.309 x 10⁻⁷ m
1 m is exactly 100 cm, so multiply this result by 100 = 10² to get a thickness of
6.309 x 10⁻⁷ m = (6.309 x 10⁻⁷ m) * (10² cm/m) = 6.309 x 10⁻⁵ cm
or 0.00006309 cm.
