Counting

How many ways can Amir, Bin, Charles, and Daria stand in a line if Amir and Bin want to stand next to each other?    

Consider Amir & Bin as a unit.  Then, how many ways can you arrange 3 items.    

The answer to that is 3 • 2 = 6    

Now have A & B swap places.  You can arrange this configuration 6 ways as well.    

Total of ways to arrange them, add both configurations:   6 + 6  =  12    

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