How much milk (in litres) is left in the vessel at the end?

Each operation removes 6/60 = 1/10 of whatever mixture is present, so the milk is multiplied by (1 − 1/10) = 9/10 each time; the total volume stays 60 L.

Milk left = 60 × (9/10)³ = 60 × 0.729 = 43.74 litres.

Formula: Final quantity = Initial × (1 − x/V)ⁿ, where x is drawn each time from volume V, repeated n times.

Simply subtracting 3 × 6 = 18 litres (giving 42) is wrong because the second and third draws remove diluted mixture, not pure milk.