This is really nitpicking but I think this can be worded a little more clearly:
"Notice that cada is invariable and is always followed by a singular noun, except when there is a number before the noun, in which case it needs a plural: " I suggest replace "it" with "the noun" . I know preposition refers to the last noun before it but in this case I was confused because I thought it meant "cada" that should be plural. I think because I assumed the noun would obviously be plural.
