When an ice box is packed with 25 identical bottles of soda, it has a mass of 17.9 kg. When the ice box is packed with 35 identical packets of coffee, it has a mass of 13.05 kg. A bottle of soda weighs 310 g more than a packet of coffee. 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 coffee = 1 P
Mass of 1 bottle of soda = 1 P + 0.31

Mass of 35 packets of coffee = 35 P
Mass of 25 bottles of soda = 25(1 P + 0.31) = 25 P + 7.75 

Ice Box + 25 P = 17.9 - 7.75 = 10.15
Ice Box + 25 P = 10.15 --- (1)

Ice Box + 35 P = 13.05 --- (2)

(2) - (1)
35 P - 25 P = 13.05 - 10.15 
10 P = 2.9

1 P = 2.9 ÷ 10 = 0.29

25 P = 25 x 0.29 = 7.25

Mass of the empty ice box
= 10.15 - 7.25
= 2.9 kg

Answer(s): 2.9 kg