Answers to last issue's Brain Teasers

1. A poor farmer went to the market to sell some peas and lentils. However, as he had only one sack and didn't want to mix peas and lentils, he poured in the peas first, tied the sack in the middle, and then filled the top portion of the sack with the lentils. At the market a rich innkeeper happened by with his own sack. He wanted to buy the peas, but he did not want the lentils. Pouring the seed anywhere else but the sacks is considered soiling. Trading sacks is not allowed. The farmer can't cut a hole in his sack. How would you transfer the peas to the innkeeper's sack, which he wants to keep, without soiling the produce? 
Ans: It's very simple. You need somewhere to "hold" the lentils while pouring out the peas. You can't pour it out, so pour the lentils into the innkeeper's sack itself. Then bind it and turn it inside out. Pour in the peas. Then unbind the sack and pour the lentils back to your sack!
2. One family wants to get through a tunnel. Dad can make it in 1 minute, mama in 2 minutes, son in 4 and daughter in 5 minutes. Unfortunately, not more than two persons can go through the narrow tunnel at one time, moving at the speed of the slower one. Can they all make it to the other side if they have a torch that lasts only 12 minutes and they are afraid of the dark?
Ans: The answer clearly is that two people go across the tunnel at a time, and one person returns with the torch every time. The trick is to pair up people who do not have very different speeds. Otherwise the faster person will be slowed down by the slower person and they all cannot make it across in time. So the first choice is to send the Mom and Dad. This takes the slower of the two times - 2 minutes.
Now it makes sense for the fastest person to return with the torch, so Dad comes back and this takes him a minute, making 3 minutes in all. Now if Dad takes one of the children back that will really slow things down so the best solution is to send both children to Mom and this takes the slower of the two times, that is, 5 minutes. The running total time is now 8 minutes.
Again, it makes sense for the fastest person to return with the torch. So Mom comes to Dad with the torch in two minutes, so 10 minutes have passed and they both come back across to their children. This takes the slower of the two times, 2 minutes, making 12 minutes in all.

3. How can you measure 6 liters of water using only 4 and 9-liter bowls?
Ans: First fill the 9-litre bowl. Then pour 4 litres into the 4-litre bowl so there are 5 litres left in the 9-litre bowl. Empty out the 4-litre bowl by throwing the water away (or keep it aside if you don't want to waste it!) and refill it from the 9-litre bowl. Since the 9-litre bowl had 5 litres, this leaves 1 litre in the big bowl. Again throw out the water in the 4-litre bowl and empty it. Then pour the remaining litre in the 9-litre bowl into the 4-litre bowl but this time keep it there. Fill the 9-litre bowl to the top for the second time and pour water to fill the 4-litre bowl to the top. Since there is already 1 litre of water in the 4-litre jug, you can pour in only 3 liters. The remaining 6 litres stays in the 9-litre bowl and this is what you needed.

From: http://brainden.com