Crossing the river, again..

At one side of river there are four men. They take different times to cross the river. Suppose A, B, C, and D are the men. A take 10 mins, B take 5 mins, C take 2 mins, D take 1 min.

The capacity of the boat is two and when two guys go in the boat, the time taken will be maximum of the two.

Suggest a way so that all four men can cross the river in the minimum possible time.

Missionaries and Cannibals.

There are three missionaries and three cannibals on one side of a river. Rhey want to cross the river on a boat which has total capacity two. But, if the number of cannibals is more than the number of missionaries, cannibals will show their true nature. i.e the life of missionaries will be at risk.

Find out a solution where they can cross the river without hurting anyone.

Twelve identical balls, one defective.

There are twelve billiards balls, and out of them one is defective in weight. You dont know if the defective ball is lighter or heavier. You are given a weigh, and asked to identify the defective ball in exactly three attempts.

The intelligent prince.

A king wants his daughter to marry the smartest of 3 extremely intelligent young princes, and so the he devised an intelligence test.

The princes are gathered into a room and seated, facing one another, and are shown 2 black hats and 3 white hats. Now They are blindfolded, and 1 hat is placed on each of their heads, with the remaining hats hidden in a different room.

Now they are asked to open their eyes.

The princes cannot see the color of their own hat. King announces that, one who identify his own hat first would win the test.

What happens next.

Jailer and prisoners.

The jailer meets with 23 new prisoners when they arrive. He tells them, "You will be in isolated cells and will have no communication with one another, not even can see or hear each other.

"In this prison is a switch room, which contains two light switches labeled 1 and 2, each of which can be in either up or the down position. I am not telling you their present positions, as they can be in any position.

"From tomorrow, from time to time whenever I feel, I will select one prisoner at random and escort him to the switch room. This prisoner will select one of the two switches and reverse its position. Then he'll be led back to his cell.

"No one else will be allowed to alter the switches until I lead the next prisoner into the switch room. I'm going to choose prisoners at random. I may choose the same guy three times in a row, or I may jump around and come back.

At any point of time, any of you can shout and tell me, "Hey, every one of us has visited the switch room at least once".

"If it is true, then you will all be set free. If it is false, you will all die horribly. You will be carefully monitored, and any attempt to break any of these rules will result in instant death to all of you"

"You will be allowed to device a strategy and discuss among yourself to night. From tomorrow the game starts"

What is the strategy they come up with so that they can be free?

Pirates and Gold coins.

There are five robbers. They steal 1000 gold coins and want to divide the coins among them: But there is caveat: The youngest robber has to present a plan to divide the coins. The plan is accepted only if majority of robbers accept it. (Two out of four won't do.)

If the plan is rejected the presenter of plan is shot and the next older robber gets a chance.

So do you think the youngest robber can save his life and get any coins? Or whats the proportion of gold coins each robber get after partition?
Note: Each robber is very intelligent.