3. There are 3 boxes. If you weigh them two at a time, their weights are 83 kg, 85 kg, and 86 kg. Question: What is the weight of the lightest box in kilograms?
3. The lightest box weighs 41 kilograms
3.【Solution】If we categorize the 3 boxes by weight as large, medium, and small, then
$$
\begin{array}{c}
83=\text { Medium + Small } \\
85=\text { Large + Small }
\end{array}
$$
$$
86=\text { Large + Medium }
$$

Therefore, the lightest box weighs $\quad(83+85-86) \div 2=41$ (kilograms)