A box contains blue marbles. Another two boxes contain only white marbles.  Label on Box $$\rm A$$: white marbles  Label on Box $$\rm B$$: blue marbles  Label on Box $$\rm C$$: Box $$\rm B$$ contains blue marbles  Which box contains blue marbles if two of the above labels are wrong? 
Overseas Competition->Knowledge Point->Combinatorics->Logical Reasoning->Reasoning using Hypothesis
If Box $$\rm A$$ has the wrong label, it contains blue marbles. This indicates that Box $$\rm B$$ has the wrong label too, because only one box has blue marbles.  It also means that Box $$\rm C$$ has the wrong label. We will then have a scenario of three wrong labels, so Box $$\rm A$$ cannot be the one having the wrong label.  If Box $$\rm A$$ has the right label, Box $$\rm B$$ and Box $$\rm C$$ will have the wrong labels.  Box $$\rm A\rightarrow$$ white marbles  Box $$\rm B\rightarrow$$ white marbles  Box $$\rm C\rightarrow$$ blue marbles  Box $$\rm C$$ contains blue marbles. 