visual - 4 sums of flags

The previous 'Sum of' puzzle can be found here

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+ =
+ =
+ =
+ = ?

Which flag should replace the question mark?

Answer

This is about the

Dialing codes of countries.

So the answer is

Australia

Explanation

1. Portugal (+351) + the Netherlands (+31) = Montenegro (+382)
2. Japan (+81) + Taiwan (+886) = Yemen (+967)
3. Algeria (+213) + Argentina (+54) = Botswana (+267)

Therefore

Egypt (+20) + Switzerland (+41) = Australia (+61)