Ah, brain teasers. We hate and love them. They are so frustratingly tricky yet if we manage to solve them, we get that “eureka!” moment that nothing else beats. Moreover, brain teasers act as our mental gym equipment to get our brains all warmed up and working. Here are 4 of the most interesting logic puzzles that you will ever encounter online, since the dawn of time.

## 1. The bridge puzzle

This puzzle is similar to the river crossing puzzle that is rumoured to be used by Japanese companies to test the IQ of their job applicants. However, this riddle comes with more constraint and as such, is harder.

Hint: How do you minimise the time the slower people take up and make full use of your faster runners?

## 2. Real and fake gold coins

There are 12 bags of 100 coins. 11 bags contain real gold coins each weighing 100 grams (so 10kg in total). Only one bag weighs 9.9kg as it contains 100 fake gold coins, weighing 99 grams each. You are given an electronic weighing scale. What is the least number of tries you need to find the bag of fake gold coins? How?

Hint: Can you find the bag of fake gold coins by just weighing once?

Answer: Number the bags from 1 to 12. Take out from the bags a corresponding number of coins. For instance, from the bag numbered 1, take out 1 coin and so on.

1+2+3….11+12= 78

You will have a total of 78 coins. If all the coins are made of real gold, the total weight should be 78×100 grams=7,800 grams. But because there are some fake gold coins each weighing 1 gram less than the real gold coins, the total weight of the 78 coins will be less than 7,800 grams. The shortfall is the number of coins. For instance, if the total weight is 7,796 grams then there are 4 fake coins, and bag 4 is the bag with the fake coins.

## 3. Head or tail

You are wearing a blindfold and thick gloves, meaning you cannot see and feel. 100 coins are laid out on a table. 20 of these coins are tails and the rest are heads. How can you can split the coins into 2 piles where the number of tails is the same in each? You are allowed to move the coins and to flip them, but you can never tell what state a coin is currently in.

Hint: Flip the coins in one pile.

Answer: Take 20 coins to form one pile. Then flip all the coins in that pile. The remaining 80 coins will form another pile.

Originally there were 20 T and 80 H on the table, right? To demonstrate, if you took 3 T and 17 H to form the small pile, the remaining 80 coins will be made up of 17 T and 63H. By flipping the smaller pile, you now get 3 H and 17 T. Tada, same number of tails in both the big and small piles. If you still do not believe me, work it out for yourself with different number of heads and tails.

## 4. One truth, one lie

You are in a room with two doors. There is a guard at each door. One door is the exit, but behind the other door is something that will kill you. You are told that one guard always tells the truth and the other guard always lies. You do not know which guard is which. You are allowed to ask one question to either of the guards to determine which door is the exit.

Hint: Find the exit, not the liar with your one question.

Answer: You cannot ask, “which door is the exit?” because both will reply “mine”. However, you can ask, “which door will the other guard say is the exit?” If you ask the guard who always tells the truth, he knows the other guard will lie, so he will point you to the door leading to death. If you ask the guard who always lies, he knows the other guard will truthfully show you the exit, so he will lie and point you to the door leading to death.

Then you just have to choose the door that both guards did not point out in order to escape.

You can find more logical thinking brain teasers online, but these 4 are the ones that will stand the test of time. Do read this article once in a while (no peeping at the answers!) to give your brain the training it deserves. Remember, you rest, you rust.