18.【Solution】The three backpacks are loaded with 8.5 kg, 6 kg, and 4 kg, 4 kg, 3 kg, and 2 kg, at this point the heaviest backpack contains 10 kg.

On the other hand, the heaviest backpack must carry no less than 10 kg: 8.5 kg must be placed alone (otherwise, the weight of this pack exceeds 10 kg). If 6 kg is placed with 2 kg, the remaining weight exceeds 10 kg. If 6 kg is placed with 3 kg, the remaining weight equals 10 kg. Therefore, the heaviest backpack contains 10 kg.
18. There are six rock specimens, weighing 8.5 kilograms, 6 kilograms, 4 kilograms, 4

kilograms, 3 kilograms, 2 kilograms. They need to be packed into three backpacks, with the requirement that the heaviest backpack should be as light as possible. Please write down the weight of the rock specimens in the heaviest backpack?