1.How many (OR: How many people/adults/children ...) are (OR: are there) in your family? ~There are five (There are five people/adults/children ...)
2. What's (OR: What's there) in your bag? ~There are two pairs of trousers, two shirts, some socks and a travel guide.
I guess #1 is an example of a double omission of subject. Am I right?
It's interesting that the answers bởi contain the impersonal subject there, although the whole construction "There are" could be omitted.
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Your second example looks lượt thích your first, but here we don't say "What's there in your bag?" (or at least it would sound rather odd).
And yet "What is there in you bag?" sounds fine.