15. Touissant L'Ouverture and George Washington wanted to know how they became the fathers of their countries and how they were able to do so without anybody's help.

This is a case of a compound noun clause.The signal word ["how"] for the second noun clause ["they were able to do so without anybody's help"] has been left out. It should be reinstated to make a sentence that makes the most sense. If it is left out, it would appear (like #10 above) that the second noun clause is masquerading as an independent clause which should be set off with a comma before the "and". If, indeed, this were the case, then the meaning of the sentence would be dramaticall changed.