Even though it seems unbelievable, cardinal numbers are not declined. It would fit into the overall picture, but still it is not done.
a) Ich gebe den vierhunderten
Frauen einen Apfel.
b) Ich gebe den vierhundert Frauen einen Apfel.
In phrase a) we declined the cardinal number, but it was only us and it is incorrect. In some few cases numbers are declined, but in general they are unchanged. Phrase b), without declension, is correct. Let's now check which the cases that have to be declined are.