English grammar

The correct sentence should be:

The value of Toronto's manufacturing sector is worth more than twice that of the four Atlantic provinces combined.

Part A of the sentence contains a mistake.
Worthy is an adjective but it is more commonly used in the following way:
Example:
The incident is worthy of your attention.

We use worth to indicate the value of an object.
Example:
How much is the jewellery worth?
It is worth over a million dollars.


Please note that "twice that" (part B) is not a phrase with "twice" and "that" together. "That" is a pronoun that stands for the value. The full sentence should be read this way:

The value of Toronto's manufacturing sector is worth more than twice the value of the four Atlantic provinces combined.

Here you must compare value with value, not value with the four provinces. Therefore, that is required to stands for the value.

The answer that he gave us seems perfect. However, something needs to be clarified first. I think Garlic's answer changed the original meaning of the sentence, and he doesn't realise that. "more than twice the value or only twice more than the value of B"????

-----Eng.Grammar------
(B)(more) worthy=worthy is adj=the value deserves to be worthy of:-ie:-
----The value of Toronto deserves to be worthy of the 4 Atlantic provinces;ie:-New Brunswick, Nova Scotia, Prince Edward, and St. John's.
(C)The findings of "twice of that" are worthy of note means "twice that"Prep."of" indicating separation in space .in description only.