Answer them. That is how to reduce unanswered stats.
If there are good questions, they deserve good answers too. So, write a good answer by yourself or let someone else by sharing who can answer better. When someone posts, vote on them. That is more important. Writing or upvoting some bad answers or any other half baked answer just to improve our stats is more harmful (applies to questions too). There was one proposal about graduation if you remember. That was solely based on stats. We know what happened to it). We are not here to write for the stats. We are here for posting quality answers. It is quality which decides the improvement of the site and not Area 51 stats alone. Stats automatically improve when we write good content.
We can write answers from many sources and not only Hindu texts. We should only write based on them. In the name of quality, many are just quoting large passages from a book/scripture without much explaining anything of their own. If the answer percentage gets to 90 with the help of those answers, they are of no use. We need answers explaining in easy terms and helping the readers. Reduce answering in comments and to and fro discussion in comments. Using that time in writing a good answer can be fruitful. There are some comments which are expanded as answers. That's totally fine.
There can be hypothetical questions which may be related to Hinduism like "Is there a mention of advent of Europeans in India in Hindu texts?". Saying Hinduism is vast and letting this open is not a good way to deal with such questions. By saying Hinduism is vast, we will not have anything to close. That is where scope comes into the picture. So, we should allow practical questions which can be answered.
When we are getting good questions, let them come. Who knows, we may get a great answer just a day before the site is shut down. That is the beauty of the Stack Exchange model. There are many instances where there are great answers posted many years after the question is posted. We just have to be patient for good answers. We actually have a lot questions where the gap between question and answer is more than an year and bumped by the Community user a couple of times. A few questions off the top of my head are as follows:
Who was Chhati Maiya?
Why does Thirumazhisai Alwar say the Pandava Thoothar statue of Krishna is older than him?
Who is the Kshetrapala god who lives on the Pariyatra mountains?