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

There are 20 people in an empty, square room. Each person has full sight of the entire room and everyone in it without turning his head or body, or moving in any way (other than the eyes). Where can you place an apple so that all but one person can see it?

What goes up but never comes down?

Can you find the key in the picture below?

I am the largest in my family of fifty.
The youngest of our family lives separately from the rest of the family and so do I.
I am regarded as harsh and tough.
I am so large, you cut me in two equal halves,
each half would still be larger than the second largest member of the family.
Who am I?

When I got four, I have it none.
When I got two, I have it some.
When I got none, I have it all.

What is it?

Which number should replace the question mark?

Can you decipher the below rebus?

PBLUESAISNUERSES

John killed hundreds of people mercilessly and yet the police were not able to put her behind the bars. Why?

If I put in one canary per cage, I have one bird too many. If I put in two canaries per cage, I have one cage too many. How many cages and canaries do I have?