What mathematical symbol can be put between 5 and 9, to get a number bigger than 5 and smaller than 9?

If I remove one from eleven it becomes Ten and if I remove one from nine also becomes Ten. How?

One is to three as three is to five and five is to four and four is the magic number.
What is the pattern?

Find the next number in the following series:
6, 14, 26, 98, __?

Which is faster, Heat or Cold ? Explain.

13 decks of cards have been mixed. What is the minimum number of cards that must be taken out from the above-mixed cards to guarantee at least one 'four of a kind?

Find the value of "?" in the ball picture Riddle below:

100 prisoners are stuck in the prison in solitary cells. The warden of the prison got bored one day and offered them a challenge. He will put one prisoner per day, selected at random (a prisoner can be selected more than once), into a special room with a light bulb and a switch which controls the bulb. No other prisoners can see or control the light bulb. The prisoner in the special room can either turn on the bulb, turn off the bulb or do nothing. On any day, the prisoners can stop this process and say "Every prisoner has been in the special room at least once". If that happens to be true, all the prisoners will be set free. If it is false, then all the prisoners will be executed. The prisoners are given some time to discuss and figure out a solution. How do they ensure they all go free?

Here is what you have to do. You have to throw a ball as hard as you can but it must return back to you even if it does not bounce at anything. Also, you have nothing attached to the ball. There is no one on the other end to catch that ball and throw it back at you.

How will you do it?

Count the number of triangles in the picture below: