In the following sentence:
Tu sais combien de secondes ça fait?
I thought "des" is needed before plural nouns (secondes), so at first I thought the following is correct:
Tu sais combien de des secondes ça fait?
But this is not correct. So my questions are:
Why is "des" not needed before "de"?
In what case is it not needed? At first I thought it is not needed before a proposition, but is this correct?
I got more confused to see the following sentence (use of des, not de):
Tous les mardis et mercredis des deux mois.
In this case, des, not de, is used.