When an ice box is packed with 15 identical bottles of coke, it has a mass of 12.5 kg. When the ice box is packed with 20 identical packets of orange juice, it has a mass of 7.6 kg. A bottle of coke weighs 390 g more than a packet of orange juice. What is the mass of the empty ice box in kg?
1000 g = 1 kg
390 g = 390 ÷ 1000 = 0.39 kg

Mass of 1 packet of orange juice = 1 P
Mass of 1 bottle of coke = 1 P + 0.39

Mass of 20 packets of orange juice = 20 P
Mass of 15 bottles of coke = 15(1 P + 0.39) = 15 P + 5.85 

Ice Box + 15 P = 12.5 - 5.85 = 6.65
Ice Box + 15 P = 6.65 --- (1)

Ice Box + 20 P = 7.6 --- (2)

(2) - (1)
20 P - 15 P = 7.6 - 6.65 
5 P = 0.95

1 P = 0.95 ÷ 5 = 0.19

15 P = 15 x 0.19 = 2.85

Mass of the empty ice box
= 6.65 - 2.85
= 3.8 kg

Answer(s): 3.8 kg