Explanation : Top tap is operational till 18 litres is drawn out.
∴ Time after which the lower tap is usually open = 18 x 6 = 108 minutes.
∴ Time after which it is open now = 108 - 24 = 84 minutes.
∴ Litres d rawn = 84/16 = 14 litres
∴ 18 - 14 = 4 litres
were drawn by the new assistant.
∴ Time = 4 x 4 = 16 minutes.
Alternatively
Upper tap was not used for 24 minutes.
∴ Litres drawn = 24/6 = 4 litres.
∴ Beer was drawn for 4x 4 = 16 minutes.