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olive
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abhijit_sen
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highhopes
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SV6
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Think that a person travels to cover all these cities.

So if he starts at City A , he will have to go to other 5 cities. So, Fill 5 dots in the row of City A to all other 5 cities.

Then from City B, he has to travel to C, D, E, F as he already travelled to A ( In the above case A -> B = B -> A) So fill 4 more entries in the Row of B to C,D, E, F..

Similarly, from City C - 3 entries

From City D - 2 entries

From City E - 1 entry

So now he has covered all the cities and if you count number of fillings in the boxes it will be 15.
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Here is the academic spirit! (+1)

highhopes
I think of it this way, where n=total number of cities. If you try to count you will be screwed when they ask you about 120 cities.

If you filled each square with a dot, that would be n*n=n^2 (# rows * # columns).
Now, you don't need a dot for A to A, B to B, and so on. So for each city that is one less dot (n^2-n).
Finally, putting a dot in every square overlaps, you have twice as many as you need because you have A to B and B to A. So you need to divide your answer by 2, giving you:

(n^2-n)/2. In this case (6^2-6)/2=15



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