Imagine you want to send a valuable item to your friend. You have big enough box on which you can place several padlocks. You both have enough padlocks but the problem is that you don’t have keys to padlocks of the other one.

How do you send the valuable item so that it doesn’t go unlocked? Sending just a key doesn’t help because someone could replicate it.

Group of friends is trapped in a burning house. The house is going to collapse in 12 minutes so they desperately want to get out. They have to run through a hallway which is in flames. If someone wants to pass through he has to have a fire extinguisher to hold the flames back at least a little. Problem is that the friends have only one.

Only two people can go at once. Than somebody has to return with the extinguisher and other two can go.

There is one fireman among the friends who moves through flames with ease and he can ran through the hallway in one minute. His best friend who is in great shape can run through in 2 minutes. There is an older man who needs four minutes and a drunken guy who will need 5 minutes to pass through. If two people go together they move at speed of the slower one.

How will they do it so that they all get out before the house collapses?

You have two ropes.

Each of them burns for one hour. They burn unevenly in different parts though – it means that one half can burn for 20 minutes and other half for 40 minutes. In addition you have a box of matches.

Measure 15 minutes!

You have 10 bags of gold coins (you wish :)).

One of your servants who were responsible for transport of the money wanted to trick you. He took one of the money bags and filed away one gram of gold from each coin. One coin normally weighs 10 grams.

Can you figure out in one scaling which bag contains lighter coins – which servant should be fired – using digital scales (shows the exact weight of an item)?

There are two cogwheels in a box. The bigger one has 24 teeth and it is tightly attached to the center of the box (it doesn’t turn or move). The smaller one has 8 teeth and it rotates around the bigger wheel. How many times does the smaller wheel turn compared with the box when it turns once around the bigger wheel?