What do you think about this introduction to answering Yes/No questions?
First answer is yes, and next answer is no:


李先生在吗？
在/不在
他是你的朋友吗？
是/不是
李先生很累吗？
累(This is shortest, but in real life people wouldn't answer like that)/不累(This is native.)
你要这张卡片吗？
要/不要
李先生有朋友吗？
有/没有


And using the 嗯 to make no answer is not the right way, people don't speak like that.