Your kid was on the right track….

½ – ¹/₃ = ¹/₆

¹/₆ = 2.5 l = 2500 cm³

½ = 2500 × 3 = 7500 cm³  (transferred to Y)

Total volume of Y -> 40 × 35 × 18 = 25200 cm³

Fraction -> ⁷⁵⁰⁰/₂₅₂₀₀ = ²⁵/₈₄