I have teeth but i don't bite. Who Am I ?

Which word does the below Plexor means ?

Name a key which is hardest to turn.

When we see 2 but call 10?

What has a ring but no finger?

John is asked to stand behind Jacob and Jacob is asked to stand behind John. Both of them feel confused about it and don't know what to do?

But it is quite possible, do you know how?

What has six faces, but does not wear makeup, has twenty-one eyes, but cannot see?

I am a ten letters word
The first four letters have the power to rule,
Next, four-letter can be eaten.
The next three-letter represents a lady.
I can fly as well

what am I looking for?

Can you find out the missing number in the grid?

The doctor advised his patient to take a tablet after every fifteen minutes. He gave him five tablets in total.

How much time will he take to have all the five tablets?