When an ice box is packed with 15 identical bottles of 100 plus, it has a mass of 10.4 kg. When the ice box is packed with 20 identical packets of milk, it has a mass of 5.4 kg. A bottle of 100 plus weighs 370 g more than a packet of milk. What is the mass of the empty ice box in kg?
1000 g = 1 kg
370 g = 370 ÷ 1000 = 0.37 kg

Mass of 1 packet of milk = 1 P
Mass of 1 bottle of 100 plus = 1 P + 0.37

Mass of 20 packets of milk = 20 P
Mass of 15 bottles of 100 plus = 15(1 P + 0.37) = 15 P + 5.55 

Ice Box + 15 P = 10.4 - 5.55 = 4.85
Ice Box + 15 P = 4.85 --- (1)

Ice Box + 20 P = 5.4 --- (2)

(2) - (1)
20 P - 15 P = 5.4 - 4.85 
5 P = 0.55

1 P = 0.55 ÷ 5 = 0.11

15 P = 15 x 0.11 = 1.65

Mass of the empty ice box
= 4.85 - 1.65
= 3.2 kg

Answer(s): 3.2 kg