When an ice box is packed with 20 identical bottles of soda, it has a mass of 19.7 kg. When the ice box is packed with 25 identical packets of green tea, it has a mass of 9.65 kg. A bottle of soda weighs 570 g more than a packet of green tea. What is the mass of the empty ice box in kg?
1000 g = 1 kg
570 g = 570 ÷ 1000 = 0.57 kg

Mass of 1 packet of green tea = 1 P
Mass of 1 bottle of soda = 1 P + 0.57

Mass of 25 packets of green tea = 25 P
Mass of 20 bottles of soda = 20(1 P + 0.57) = 20 P + 11.4 

Ice Box + 20 P = 19.7 - 11.4 = 8.3
Ice Box + 20 P = 8.3 --- (1)

Ice Box + 25 P = 9.65 --- (2)

(2) - (1)
25 P - 20 P = 9.65 - 8.3 
5 P = 1.35

1 P = 1.35 ÷ 5 = 0.27

20 P = 20 x 0.27 = 5.4

Mass of the empty ice box
= 8.3 - 5.4
= 2.9 kg

Answer(s): 2.9 kg