You have two sand hourglasses, one that measures exactly 4 minutes and one that measures exactly 7 minutes. You need to measure out exactly 2 minutes to boil an egg. Using only these two hourglasses, how can you measure out exactly 2 minutes to boil your egg?
Flip over both hourglasses at the same time.
1. After 4 minutes, the 4-minute hourglass will be done, and there will be 3 minutes left in the 7-minute hourglass. Immediately flip the 4-minute hourglass over again.
2. After 3 more minutes, the 7-minute hourglass will be done, and there will be exactly 1 minute left in the 4-minute hourglass. Immediately flip the 7-minute hourglass over again.
3. After 1 more minute, the 4-minute hourglass will be done again, and there will be exactly 6 minutes left in the 7-minute hourglass. Immediately flip over the 4-minute hourglass.
4. After 4 more minutes, the 4-minute hourglass will be done again, and there will be exactly 2 minutes left in the 7-minute hourglass. At this point, put your egg in the boiling water. When the 7-minute hourglass is done, it will have been exactly 2 more minutes, and your egg will have boiled just right.
Or after step 2 just flip 7-minute hourglass for second minute.
I am the black child of a white father, a wingless bird, flying even to the clouds of heaven. I give birth to tears of mourning in pupils that meet me, even though there is no cause for grief, and at once on my birth I am dissolved into air. What am I?
There are three houses. One is red, one is blue, and one is white. If the red house is to the left of the house in the middle, and the blue house is to the right to the house in the middle, where is the white house?
A crime has been committed at Freemont Street. The main suspect is a man named Sean Baker. It was said that a man had been walking along the pathway when he was suddenly shot in the stomach. The suspect had brown hair, blue eyes and wore a baggy Armani suit just like Sean Baker's. Sean was asked to tell the story right from the beginning. "Well," said Sean, "I was just hanging around the park when I saw this man walking along the pathway. Suddenly, a guy came up from behind him and shot him! I ran home as fast as I could. The policemen asked him to give a description of the murderer. "He had a red mustache, red hair and a baggy Armani suit on."
"I think this man is telling a lie," said one of the policemen. How did he know?
How can the murderer shoot him in the stomach if he came up behind the man?
A frog is at the bottom of a well. It is a 30 foot climb to get out. Each morning, the frog jumps 3 feet up the path out, but each night, as it sleeps, it slips back 2 feet down.
Thus, at the beginning of the first day, the frog has 30 feet to go, at the beginning of the second day it has 29 feet to go, and so on.
How many days does it take the frog to get out of the well?
It takes 28 days for the frog to get out (it gets out on the morning of the 28th day).
This is because on the beginning of the 28th day, the frog has 3 feet left to travel. Because the frog jumps 3 feet forward each morning, it will jump out of the well on this day.
A couple went on for a climbing trip. But only the husband returned from the vacation and said that his wife slipped off while climbing and died. On investigating, the local sheriff arrested him saying, 'Your travel agent called. You murdered your wife.'
The man did not inform anyone about the trip. Then how did the agent was so sure that it is a murder?
The man bought only one way ticket for his wife whereas he bought two way ticket for himself. It means that he was sure that he will be returning alone.
An intelligent trader travels from one place to another with 3 sacks having 30 coconuts each. No sack can hold more than 30 coconuts. On the way, he passes 30 check points. At each check point, he has to give one coconut for every sack he is carrying. What is the maximum number of coconuts that he can have with him at the end of his journey?
He will have 25 coconuts with him at the end. The trick is to reduce the number of sacks as you pass checkpoints. The first 10 checkpoints require 3 coconuts each, which empties his first sack. The next 15 checkpoints require 2 coconuts each, which will empty his second stack. Now, he is left with 1 sack and 5 more checkpoints. So, the 5 checkpoints will take 1 coconut each. Therefore, he will be left with 25 coconuts.
A man was shot to death while in his car. There were no powder marks on his clothing which indicated that the gunman was outside the car. However, all the windows were up and the doors locked. After a close inspection was made, the only bullet holes discovered were on the man's body. How was he murdered?
You and a friend are standing in front of two houses. In each house lives a family with two children.
"The family on the left has a boy who loves history, but their other child prefers math," your friend tells you.
"The family on the right has a 7-year old boy, and they just had a new baby," he explains.
"Does either family have a girl?" you ask.
"I'm not sure," your friend says. "But pick the family that you think is more likely to have a girl. If they do have a girl, I'll give you $100."
Which family should you pick, or does it not matter?
You should pick the house on the left. Specifically, there is a 2/3 chance that the family on the left has a girl, whereas there's only a 1/2 chance that the house on the right has a girl.
This is a very counterintuitive riddle. It seems like there should always be a 1/2 chance that a given child is a girl. And in fact there is. The key word there is "given". Because we are not asking about a "given" child for the house on the left. We are asking about what could be either child. Whereas for the house on the right, we are asking about a "given" child...specifically, we're asking about the younger child.
There are 3 possibilities for the children in the first house:
There is no "Girl, Girl" option because we know the house on the left has at least one boy. Since each of these 3 options is equally likely, and 2 of them have one girl, there is a 2/3 chance of there being a girl in the house on the left.
For the house on the right, because we already know the older child is a boy, there are only two possibilities:
And as we can see, there is a 1/2 chance for the house on the right having a girl.
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