If you remove one from eleven, it becomes ten. If you remove one from nine, it becomes ten.

How is this possible?

Mr Black, Mr Gray, and Mr White are fighting in a truel. They each get a gun and take turns shooting at each other until only one person is left. Mr Black, who hits his shot 1/3 of the time, gets to shoot first. Mr Gray, who hits his shot 2/3 of the time, gets to shoot next, assuming he is still alive. Mr White, who hits his shot all the time, shoots next, assuming he is also alive. The cycle repeats. If you are Mr Black, where should you shoot first for the highest chance of survival?

A bag contains 64 balls of eight different colours. There are eight of each colour (including red). What is the least number you would have to pick, without looking, to be sure of selecting 3 red balls?

On rolling two dices (six-sided normal dice) together, what is the probability that the first one comes up with a 2 and the second one comes up with a 5?

What four weights can be used to balance from 1 to 30 pounds?

Count the number of triangles in the picture below:

Can you count the number of triangles in the given figure?

Can you count the number of squares in the given picture?

Take away my first letter, then take away my second letter. Then take away the rest of my letters, yet I remain the same. What am I?

I am a prime number.
The double of myself is equal to the square of me.

which number am I?