When an ice box is packed with 20 identical bottles of soda, it has a mass of 13.2 kg. When the ice box is packed with 25 identical packets of green tea, it has a mass of 8.2 kg. A bottle of soda weighs 310 g more than a packet of green tea. What is the mass of the empty ice box in kg?
1000 g = 1 kg
310 g = 310 ÷ 1000 = 0.31 kg

Mass of 1 packet of green tea = 1 P
Mass of 1 bottle of soda = 1 P + 0.31

Mass of 25 packets of green tea = 25 P
Mass of 20 bottles of soda = 20(1 P + 0.31) = 20 P + 6.2 

Ice Box + 20 P = 13.2 - 6.2 = 7
Ice Box + 20 P = 7 --- (1)

Ice Box + 25 P = 8.2 --- (2)

(2) - (1)
25 P - 20 P = 8.2 - 7 
5 P = 1.2

1 P = 1.2 ÷ 5 = 0.24

20 P = 20 x 0.24 = 4.8

Mass of the empty ice box
= 7 - 4.8
= 2.2 kg

Answer(s): 2.2 kg