When an ice box is packed with 15 identical bottles of coke, it has a mass of 12.85 kg. When the ice box is packed with 20 identical packets of lemonade, it has a mass of 6.5 kg. A bottle of coke weighs 510 g more than a packet of lemonade. What is the mass of the empty ice box in kg?
1000 g = 1 kg
510 g = 510 ÷ 1000 = 0.51 kg

Mass of 1 packet of lemonade = 1 P
Mass of 1 bottle of coke = 1 P + 0.51

Mass of 20 packets of lemonade = 20 P
Mass of 15 bottles of coke = 15(1 P + 0.51) = 15 P + 7.65 

Ice Box + 15 P = 12.85 - 7.65 = 5.2
Ice Box + 15 P = 5.2 --- (1)

Ice Box + 20 P = 6.5 --- (2)

(2) - (1)
20 P - 15 P = 6.5 - 5.2 
5 P = 1.3

1 P = 1.3 ÷ 5 = 0.26

15 P = 15 x 0.26 = 3.9

Mass of the empty ice box
= 5.2 - 3.9
= 1.3 kg

Answer(s): 1.3 kg