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A set of ten cards, each showing one of the digits from $0$ to $9$, is divided up between five envelopes so that there are two cards in each envelope. The sum of the two numbers inside it is written on each envelope:

five envelopes with the numbers 7, 8, 13, 14 and 3 written on them (one number per envelope)

What numbers could be inside the "8" envelope?

Thank you to Alan Parr for allowing us to adapt one of his problems.