When an ice box is packed with 20 identical bottles of soda, it has a mass of 17.1 kg. When the ice box is packed with 25 identical packets of coffee, it has a mass of 7.8 kg. A bottle of soda weighs 510 g more than a packet of coffee. 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 coffee = 1 P
Mass of 1 bottle of soda = 1 P + 0.51

Mass of 25 packets of coffee = 25 P
Mass of 20 bottles of soda = 20(1 P + 0.51) = 20 P + 10.2 

Ice Box + 20 P = 17.1 - 10.2 = 6.9
Ice Box + 20 P = 6.9 --- (1)

Ice Box + 25 P = 7.8 --- (2)

(2) - (1)
25 P - 20 P = 7.8 - 6.9 
5 P = 0.9

1 P = 0.9 ÷ 5 = 0.18

20 P = 20 x 0.18 = 3.6

Mass of the empty ice box
= 6.9 - 3.6
= 3.3 kg

Answer(s): 3.3 kg