Page 99 of Hindry's Arithmetics, follows from exact sequence that $\text{N}(IJ) = \text{N}(J)\text{card}(J/IJ)$?

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Considering the following excerpt from page 98-99 of Hindry's Arithmetics.

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Why does it follow from the exact sequence that $\text{N}(IJ) = \text{N}(J)\text{card}(J/IJ)$? Thanks in advance.

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In an exact sequence $$0\to A\to B\to C\to 0 $$of abelian groups we have $|B|=|A|\cdot |C|$.