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I'll try to explain with an example:
Lets say you have four countries with amount of A, B, C and D people respectively. Now we should calculate the number of posible pairs from different countries. The order does't matter so the calculation would be:
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I'll try to explain with an example: Lets say you have four countries with amount of A, B, C and D people respectively. Now we should calculate the number of posible pairs from different countries. The order does't matter so the calculation would be:
A*B + A*C + A*D + B*C + B*D + C*D = A*B + (A+B)*C + (A+B+C)*D
From that pattern it is easy to see the solution proposed above.