This Problem is HCF related problem
Prime factorisation of 80 = 2 × 2 × 2 × 2 × 5
Prime factorisation of 100 = 2 × 2 × 5 × 5
Prime factorisation of 120 = 2 × 2 × 2 × 3 × 5
Product of common factors 80, 100 and 120 = 2 × 2 × 5 = 20
HCF (80, 100, 120) = 20
The volume of the vessel = 20 litres