Difficulty ★★★★☆ Popularity ★★★★★

Four people need to cross a dark river at night.They have only one torch and the river is too risky to cross without the tourch. if all people cross simultanoesly then torch light wont be sufficient.Speed of each person of crossing the river is different.cross time for each person is 1 min, 2 mins, 7 mins and 10 mins. What is the shortest time needed for all four of them to cross the river ?

Difficulty ★★★★☆ Popularity ★★★★☆

Mr. Black, Mr. Gray, and Mr. White are fighting in a truel. They each get a gun and take turns shooting at each other until only one person is left. Mr. Black, who hits his shot 1/3 of the time, gets to shoot first. Mr. Gray, who hits his shot 2/3 of the time, gets to shoot next, assuming he is still alive. Mr. White, who hits his shot all the time, shoots next, assuming he is also alive. The cycle repeats. If you are Mr. Black, where should you shoot first for the highest chance of survival?

Difficulty ★★★☆☆ Popularity ★★★★☆

There is a bag which have 21 blue balls and 23 red balls. You also have 22 red balls outside the bag.

Randomly remove two balls from the bag.

* If they are of different colors, put the blue one back in the bag.

* If they are the same color, take them out and put a red marble back in the bag.

Repeat this until only one marble remains in the bag.

What is the color of the sole marble left in the bag ?

Difficulty ★★★☆☆ Popularity ★★★★★

Time in a digital clock can be palindromic like 12:12 (same when read forwards or backwards).

Whats the minimum interval between 2 times that are palindromic ?

Difficulty ★★★★★ Popularity ★★★☆☆

The puzzle question is : On Bagshot Island, there is an airport. The airport is the homebase of an unlimited number of identical airplanes. Each airplane has a fuel capacity to allow it to fly exactly 1/2 way around the world, along a great circle. The planes have the ability to refuel in flight without loss of speed or spillage of fuel. Though the fuel is unlimited, the island is the only source of fuel.

What is the fewest number of aircraft necessary to get one plane all the way around the world assuming that all of the aircraft must return safely to the airport? How did you get to your answer?

Notes:

(a) Each airplane must depart and return to the same airport, and that is the only airport they can land and refuel on ground.

(b) Each airplane must have enough fuel to return to airport.

(c) The time and fuel consumption of refueling can be ignored. (so we can also assume that one airplane can refuel more than one airplanes in air at the same time.)

(d) The amount of fuel airplanes carrying can be zero as long as the other airplane is refueling these airplanes. What is the fewest number of airplanes and number of tanks of fuel needed to accomplish this work? (we only need airplane to go around the world)

Difficulty ★★★☆☆ Popularity ★★★★☆

A worker is to perform work for you for seven straight days. In return for his work, you will pay him 1/7th of a bar of gold per day. The worker requires a daily payment of 1/7th of the bar of gold. What and where are the fewest number of cuts to the bar of gold that will allow you to pay him 1/7th each day?

Difficulty ★★★★★ Popularity ★★★★☆

There are 100 bulbs in a room. 100 strangers have been accumulated in the adjacent room. The first one goes and lights up every bulb. The second one goes and switches off all the even numbered bulbs – second, fourth, sixth... and so on. The third one goes and reverses the current position of every third bulb (third, sixth, ninth… and so on.) i.e. if the bulb is lit, he switches it off and if the bulb is off, he switches it on. All the 100 strangers progresses in the similar fashion.

After the last person has done what he wanted, which bulbs will be lit and which ones will be switched off ?

Difficulty ★★★☆☆ Popularity ★★★★☆

There are 3 bags and all are labeled incorrectly as follow:

Bag1 : Silver

Bag2 : Gold

Bag3 : Silver and Gold.

You need to label all bags correctly by opening just one bag.

How come ?

Difficulty ★★★☆☆ Popularity ★★★★☆

Find three whole, positive numbers that have the same answer when multiplied together as when added together.

Difficulty ★★★☆☆ Popularity ★★★★☆

What is the probability of getting five Monday in a 31-days month ?