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

There is a box in which distinct numbered balls have been kept. You have to pick two balls randomly from the lot.

If someone is offering you a 2 to 1 odds that the numbers will be relatively prime, for example
If the balls you picked had the numbers 6 and 13, you lose $1.
If the balls you picked had the numbers 5 and 25, you win $2.

Will you accept that bet?

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

Can it be possible to build a house that has all its four walls facing towards the South?

If you toss a coin 10 times and it lands heads up every time, what are the chances it will land heads up if you toss it again?

What does the below rebus puzzle mean?

Find the next number in the series below.

6 18 270 72090 ?

It's pretty hard to give up.
If you remove a part of it, you will be left with a bit.
Even if you remove another part, the bit still remains.
Remove one more and it still remains.

What is it?

The chance of Mr John winning the lottery is 10%. All participants lined up and Mr John is 4th in the row. The first three participants lose the lottery.

What is the chance of Mr John now?

What's a single-digit number with no value?