What is the next number in this matchsticks series riddle?

Take 9 from 6, 10 from 9, 50 from 40, and leave 6.

How Come ??

If 5 cats catch 5 mice in 5 minutes, how long will it take one cat to catch a mouse?

I have two coins.
* One of the coins is a faulty coin having a tail on both sides of it.
* The other coin is a perfect coin (heads on side and tail on other).

I blindfold myself and pick a coin and put the coin on the table. The face of the coin towards the sky is the tail.

What is the probability that another side is also tail?

What 4-letter word can be written forward, backward or upside down, and can still be read from left to right?

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

Why do you find that most of the buildings are deprived of the 13th floor?

The host of a game show offers the guest a choice of three doors. Behind one is an expensive car, but behind the other two are goats.
After you have chosen one door, he reveals one of the other two doors behind which is a goat (he wouldn't reveal a car).

Now he gives you the chance to switch to the other unrevealed door or stay at your initial choice. You will then get what is behind that door.

You cannot hear the goats from behind the doors, or in any way know which door has the prize.

Should you stay, or switch, or doesn't it matter?

Count the number of triangles in the below picture?

What has one letter and starts with an E?