The previous 'Sum of' puzzle can be found here
Which flag should replace the question mark?
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This is about the
Dialing codes of countries.
So the answer is
1. Portugal (+351) + the Netherlands (+31) = Montenegro (+382)
2. Japan (+81) + Taiwan (+886) = Yemen (+967)
3. Algeria (+213) + Argentina (+54) = Botswana (+267)
Egypt (+20) + Switzerland (+41) = Australia (+61)