Interesting Fact of Handshake Count

Suppose we fill Yankee Stadium with 50,000 people and ask them to spend the day shaking hands with one another.

Prove that, at the end of the day, at least two participants will have shaken hands with the same number of people.

Click here for proof!

Proving Interesting Fact of Handshake Count

What was that fact?

Let's contradict the given fact & assume no 2 people have same number of handshakes. In that case, the most gregarious person would have 49999 handshakes & next gregarious person would shake hands with 49998 people and so on. 

This way, the shyest person should have 0 shake hands. But the most gregarious guy must have had handshake with this shyest guy as his count of 49999 also includes this shyest guy. So this is impossible case.

Hence, at least 2 participants would have shaken hands with the same number of people.

 
In other way, the most shyest participant would have 1 handshake, next shyer guy would have 2 & so on. The more gregarious would have 49999 handshakes that includes the shake hand with the most gregarious person. Now, most gregarious person is bound to have 49999 handshakes as he/she can't have 50000 as there are only 50000 people in the stadium including himself/herself. 

That's why, at least 2 participants would have shaken hands with the same number of people.

How Far Did I Run?

I leave my front door, run on a level road for some distance, then run to the top of a hill and return home by the same route. I run 8 mph on level ground, 6 mph uphill, and 12 mph downhill. 

If my total trip took 2 hours, how far did I run?

I ran the distance.....Click here for more!

Calculation of Avarage Speed is Tricky!

First read what was the question!

Let's first find my average speed when I was running uphill & downhill.

Assume 'x' be the distance that I have to run to reach at the top of the hill in time 'y'.

So x/y = 6 mph.

While running downhill, I cover same distance 'x' in time 'y/2' as I ran at double speed of 12 mph.

Average speed = Total Distance / Total Time

Average speed = (x + x) / (y + y/2)

Average speed = (4/3)(x/y)

Average speed = (4/3) x 6

Average speed = 8 mph.

That means my average speed on hill is equal to the my speed on level ground and that is 8 mph.

Since I ran for 2 hours in my trip the distance I ran is 8 x 2 = 16 miles.

A Long Journey of 27000 Miles

The MacDonalds are planning a long car journey of 27,000 miles. If they use tires that last 12,000 miles each, how many tires will they need, and how can they make the best use of them?

This is how usage of tires can be optimized!

Optimizing The Use of Tires in Long Journey!

How long the actual journey is?

Since each tire would be traveling 27,000 miles, when car travels 27000 miles; tire -miles are equal to 27000 x 4 = 108000.

But the each tire lasts for 12000 then tires required = 108000/ 12000 = 9.

Now managing use of these 9 tires need some planning.

For first 12000 miles of 27000 miles they can use 4 out of 9 tires. For the rest of 15000 miles use of remaining 5 tires need to be planned.

Best way is to change 1 tire after every 3000 miles like below.

First 3000 miles: Tires 1, 2, 3, 4

Second 3000 miles: Tires 2, 3, 4, 5

Third 3000 miles: Tires 3, 4, 5, 1

Fourth 3000 miles: Tires 4, 5, 1, 2

Fifth 3000 miles: Tires 5, 1, 2, 3

This way, it's made sure that each tire is used for only 12000 miles. And this is how the journey can be completed using 9 tires only.

The Race Between 2 Brothers

Zachary challenges his brother Alexander to a 100-meter race. Alexander crosses the finish line when Zachary has covered only 97 meters.

The two agree to a second race, and this time Alexander starts 3 meters behind the starting line.

 
If both brothers run at the same speed as in the first race, who will win?

He will win the second race for sure! 

Source 

Winner of The Race Between 2 Brothers

What was the race of?

Let's assume that, Alexander completes the first race in time 't'. That means, he reaches at the finish line after running 100m after time 't' since start of the race. In the same time interval, Zachary could reach only 97m.

Now, in second race too, Alexander covers 100m once again in time interval 't' & Zachary runs 97m distance. Since, Alexander started 3m ahead of start line, at this point of time both are at the same point with 3m left to complete the race. 

 
Since, Alexander had won first race with faster speed & speed of both are unchanged in second race, it's clear that Alexander will take less time to cover leftover 3m distance. Hence, Alexander will be winner of the second race. 

MATHEMATICAL APPROACH:

Let's suppose Alexander took 10s to complete the first race. Then, his speed is 100/10 = 10m/s.

In 10s, the Zachary could run only 97m. So, his speed is 97/10 = 9.7m/s.

In the second race, their respective speeds are unchanged but Alexander has to run 103m to reach at the finish line compared to 100m of Zachary.

Hence, time taken by Alexander to reach at the finish line = 103/10 = 10.3s and that taken by Zachary = 100/9.7 = 10.30929s.

It's clear that Zachary needs more time to finish this race too. Hence, Alexander will be the winner in this race as well. He beats Zachary by 100 - (10.3x9.7) = 0.09m.

Truth Tellers and Liars in Circle

On a certain island there live only knights, who always tell the truth, and knaves, who always lie.

One day you find 10 island natives standing in a circle. Each one states: "Both people next to me are knaves!"

Of the 10 in the circle, what is the minimum possible number of knights?

Do you think there can be 5?

Identifying Number of Truth Tellers in Circle

What was the task given?

Recalling the given situation. 

On a certain island there live only knights, who always tell the truth, and knaves, who always lie.

One day you find 10 island natives standing in a circle. Each one states: "Both people next to me are knaves!"

 
Every Knight must be surrounded by 2 Knaves and every Knave has to be surrounded by at least one knight to satisfy the given condition. So there must be Knave-Knight-Knave groups must be standing in circle. 

Now if Knave of previous group is counted for the next group, then there will be 5 knights in the circle as shown below.

But the question asks minimum possible number of Knights in the circle.

So after forming 3 groups of Knave-Knight-Knave separately (total 9 in circle), the last person will be obviously surrounded by 2 knaves. Hence, he must be Knight. See below.

This way there can be only 4 knights standing in the circle without violating the given condition.
 

Cars Around Interesting Circular Track

Around a circular race track are n race cars, each at a different location. At a signal, each car chooses a direction and begins to drive at a constant speed that will take it around the course in 1 hour. When two cars meet, both reverse direction without loss of speed. Show that at some future moment all the cars will be at their original positions.

Here is that proof!


Source 

Analysing Interesting Circular Race Track

What was the interesting fact about?

Just imagine that each car carries a flag on it and on meeting pass on that flag to the next car. Obviously, this flag will move at the constant speed around the track as cars carrying it are also moving at the constant speed. So, the flag will be back at the original position after 1 hour.

Let's assume there are only 2 cars on the track at diagonally opposite points as shown below. 

After 15 minutes, on meeting with Car 2, Car 1 will pass on the flag & both will reverse their own direction. 30 minutes later (i.e. 45 minutes after start) both cars again meet each other and Car 2 will pass on flag back to Car 1 & directions are reversed again. Again in another 15 minutes (i.e. after 1 hour from start), both cars are back at the original positions. 

Now, let's suppose that there are 4 cars on the track positioned as below.

The above image shows how cars will be positioned after different points of time & how they reverse direction after meeting.

Again, all are back to the original position after 1 hour including the flag position. One more thing to note that the orders in which cars are never changes. It remains as 1-2-3-4 clockwise. 

To conclude, for 'n' number of cars, at some point of time all the cars will be in original positions in future.   
 

Geometrical Puzzle

Find the area of the shaded region.

Escape to the answer! 


Source 

Geometrical Puzzle - Solution

What was the puzzle?

Let's draw a line from each of vertex to the point at which all 4 regions intersect. This divides the given area into 2 triangles as shown below.

Obviously, here A and A' have equal area as they both share same base QS and height OT. Similarly, the areas of B & B', areas of C & C' and areas of D & D' must be equal. 

Rewriting, A = A', B = B', C = C' and D = D'.

 
Now rewriting respective areas,

It's clear that,

A + B = 32

C + D = 16

Adding above 2 equations gives, 

A + B + C + D = 32 + 16 = 48

But from figure, B + C = 20,

A + 20 + D = 48

A + D = 28.

That's the area of the shaded region which is equal to 28 Sq.cm

Whose Number is Bigger?

Ali and Zoe reach into a bag that they know contains nine lottery balls numbered 1-9. They each take one ball out to keep and they look at it secretly. Then, they make the following statements, in order:

Ali: "I don't know whose number is bigger." 

Zoe: "I don't know whose number is bigger either." 

Ali: "I still don't know whose number is bigger." 

Zoe: "Now I know that my number is bigger!"

Assuming Ali and Zoe are perfectly logical, what is Zoe's smallest possible number?

'This' is that smallest possible number!

My Number is the Bigger One!

First you can read what happened?

Recalling what Ali and Zoe said - 

Ali: "I don't know whose number is bigger." 

Zoe: "I don't know whose number is bigger either." 

Ali: "I still don't know whose number is bigger." 

Zoe: "Now I know that my number is bigger!" 

First statement of Ali indicates that she doesn't have either 1 or 9. If she had 1 (or 9) then she would have an idea that Zoe must have bigger (or smaller) number.

Now Zoe is smart enough to know that Ali doesn't have 1 or 9 which is clear from Ali's first statement. Zoe's first statement indicates that she doesn't have 2 or 8 (& neither 1 or 9). If she had 1/2 (or 8/9) then she could have concluded that Ali has bigger (or smaller) number.

Till now Ali has an idea that the Zoe doesn't have 1,2,8,9. So still Ali can't have 2/8 as in that case too she could have made a different statement. Further if Ali had 3 or 7 (and knowing the fact that Zoe doesn't have 1,2,8,9); Ali could have an idea whose number is bigger as 3 is smallest while 7 is biggest among remaining numbers. That means she doesn't have 3 or 7 ( and 1,2,8,9).

From all the statement Zoe can conclude that Ali doesn't have 1,2,3,7,8,9. In short, Ali must have either 4,5,6. 

Now when Zoe says she has bigger number then it must be either 6 or 7 and Ali having 4 or 5. It can't be 5 as in that case Zoe wouldn't be confident as Ali could have 6.

So the Zoe's smallest possible number is 6.

     

The Unfair Arrangement!

Andy and Bill are traveling when they meet Carl. Andy has 5 loaves of bread and Bill has 3; Carl has none and asks to share theirs, promising to pay them 8 gold pieces when they reach the next town.

They agree and divide the bread equally among them. When they reach the next town, Carl offers 5 gold pieces to Andy and 3 to Bill.

“Excuse me,” says Andy. “That’s not equitable.” He proposes another arrangement, which, on consideration, Bill and Carl agree is correct and fair.

How do they divide the 8 gold pieces?

This is fair arrangement of gold distribution! 

Source 

Correcting The Unfair Arrangement!

How unfair the arrangement was?

First we need to know how 8 loaves (5 of Andy & 3 of Bill) are equally distributes among 3.

If each of them is cut into 2 parts then total 16 loaves would be there which can't be divided equally among 3.

Suppose, each of loaves is divided into 3 parts making total 24 loaves available.

Now, Andy makes 15 pieces of his 5 loaves. He eats 8 and gives the remaining 7 to Carl.

Bill makes 9 pieces of his 3 loaves. He eats 8 and gives the remaining 1 to Carl.

This way, Carl too gets 8 pieces and 8 breads are distributed equally among 3.

 
Obviously, Carl should pay 7 gold pieces to Andy for his 7 pieces and 1 gold piece to Bill for the only piece offered by Bill.