Can you replace the question mark with the correct number?

When can we add 2 to 11 and get 1 as the correct answer?

If eleven plus two equals one, what does nine plus five equal?

John is found dead in a bare field. Nobody killed John,
John did not commit suicide.

Inspector Jacob found a bag just a few meters away from John which could save his life. What was in the bag?

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

Decode below rebus
D
N
A
T
S , D

How can you write nineteen in a manner that if we take out one, it becomes twenty?

I throw two dice simultaneously.

What is the probability of getting a sum as 9 of the two numbers shown?

A professor thinks of two consecutive numbers between 1 and 10.
'A' knows the 1st number and 'B' knows the second number

A: I do not know your number.
B: Nor do I know your number.
A: Now I know.

What are the four solutions for this?

I get hot but I never sweat,
I cook food but I am not a chef,
I have a door but you can't enter me,
I have a light but I am not a lamp.
Who am I?