They are both correct. Each is asking a slightly different question. A. "How different are they ? Generally speaking, this is more a question of "how much" different they are. For example: Question: How different are your two sisters ? Answer: Not much. They are very similar in personality. But when you ask ... B) How are they different? Here you asking the specifics of how different they are. Using the example from above: Question: How are they different (your sisters) ? Answer: One is an extrovert, the other is very quiet.

They are both correct, but they have different meanings. 1. "How different are they?" You'd say this if you want to know the degree of difference. Likely answers are 'They're almost the same' or 'They're similar in some respects but different in others ' or 'They're very different indeed'. 2. "How are they different?" You'd say this if you want details of the precise difference. You'd expect the person to compare sizes, shapes, uses and so on.

"How are they different" is correct .