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Inverted cards puzzle

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inverted cards puzzle

inverted cards puzzle

One fine day, Santa and banta were playing cards, but suddenly power went off and they were getting bored. So santa randomly inverted position of 10 cards out of 52 cards(and shuffled it) and asked banta to divide the card in two pile with equal number of inverted cards(number of cards in each pile need not be equal). It was very dark in the room and banta could not see the cards, after thinking a bit banta divided the cards in two piles and quite surprisingly on counting number of inverted cards in both the piles were equal.
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Four glasses on a square table

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four glasses on a square table puzzle, also known as the blind bartender’s problem

Four glasses are placed on the corners of a square table. Some of the glasses are upright (up) and some upside-down (down). You have to arrange the glasses so that they are all up or all down (while keeping your eyes closed all the time). The glasses may be re-arranged in turns subject to the following rules.

  1. Any two glasses may be inspected in one turn and after feeling their orientation you may reverse the orientation of either, neither or both glasses.
  2. After each turn table is rotated through a random angle.
  3. At any point of time if all four glasses are of the same orientation a ring will bell

You have to come up with a solution to ensure that all glasses have the same orientation (either up or down) in a finite number of turns. The algorithm must be non-stochastic i.e. it must not depend on luck.
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Days of month using 2 dice

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How can you represent days of month using two 6 sided dice ?

You can write one number on each face of the dice from 0 to 9 and you have to represent days from 1 to 31, for example for 1, one dice should show 0 and another should show 1, similarly for 31 one dice should show 3 and another should show 1.

represent days of month using two dice

represent days of month using two dice

See Answer
Answer is:
Dice 1: 0 1 2 3 5 7
Dice 2: 0 1 2 4 6 8

How?
Basically you have to show 11, 22 so 1 and 2 should be present in both dices, similarly to show 01, 09 0 should be present in both dices, now the trick is for showing 9 you can use dice with 6 printed on one of the face.

How many people in party?

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At a party, everyone shook hands with everybody else. There were 66 handshakes. How many people were at the party?
See Solution: How many people in party?

lets say there are n persons
first person shakes hand with everyone else: n-1 times(n-1 persons)
second person shakes hand with everyone else(not with 1st as its already done): n-2 times
3rd person shakes hands with remaining persons: n-3

So total handshakes will be = (n-1) + (n-2) + (n-3) +…… 0;
= (n-1)*(n-1+1)/2 = (n-1)*n/2 = 66
= n^2 -n = 132
=(n-12)(n+11) = 0;
= n = 12 OR n =-11
-11 is ruled out so the answer is 12 persons.

Divide a triangle into n triangles

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How to divide a triangle into n equal area triangles ?
See Solution: Divide a triangle into n triangles

For any triangle its area can be calculated using the formula base * height/2. So we can divide a triangle into n triangles of equal area by simply dividing the base into n equal parts.

Explanations :
For example to divide the triangle in the image below into 4 equal triangles, we can simply divide its base of 8cms to 4 equal parts of 2cms each and form the 4 triangles of equal area.

divide a triangle into n triangles

divide a triangle into 4 triangles of equal area

Red and blue balls

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You have three bags and three labels. One bag has only red balls, one has only blue balls and one has both red and blue balls. Three labels are R, B and RB. R label was meant for the bag with only red balls, B label was meant for the bag with only blue balls and RB for the bag with both red and blue balls. Ram by mistake labelled the bags wrongly such that all the labels are wrong, how many minimum number of balls he should pick and from what bags to correct the labels? It is given that each bag has unlimited number of balls to be picked.

See Solution : Red and blue balls
The minimum number of balls he should pick is 1 from the bag with label RB.

How ?

As the label of all the bags are wrong, bag with label RB can either have all red balls or all blue balls.

Now ram will either get red ball or the blue ball.

Case 1: Ram gets the red ball
Bag with label RB is the bag with all the red balls(R).
Bag with label B can either be RB or R(as all the labels are wrong) but as we already know R bag so it can only be RB.
We know 2 bags correctly so third bag with label R should be B.

Case 2: Ram gets the blue ball
Bag with label RB is the bag with all the blue balls(B).
Bag with label R can either be B or RB(as all the labels are wrong) but as we already know B bag so it can only be RB.
we know 2 bags correctly so third bag with label B should be R.

Heavier ball

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You have 8 balls all look identical (in shape, color etc.). All of them have same weight except one defective ball which is heavier than others. You can use a two sided balance system (not the electronic one). Find the minimum no. of measures required to separate the defective ball and the way you separate it.

If you have solved it then try it for N no. of identical balls with one defective ball.

See Solution : Heavier Ball

The minimum no. of measures required are 2. Now can you solve it… ? Give it a try…

Here is the solution which tells you that how can you do it in 2 measurements.

Divide 8 balls into groups of 3, 3 and 2.

First weigh :
Weigh the two groups of 3 ball, now there are two possibilities
a) They are balanced (all 6 balls are of equal weight)
b) One side is heavier then the other.

case (a) :
The group of 2 has the defective ball. Weigh them with one on each side of balancing machine. The side which has more weigh has the heavier/defective ball.

case (b) :
We got the three balls of the side which is heavier. Now take any two balls and weigh them, then again there are two cases.
(i) They both are of equal weight.
(ii) One is heavier than the other.

for case (b)(i) :
The third ball is heavier/defective than all other balls

for case (b)(ii) :
The heavier ball is defective.
——————————————–

Solution for N no. of balls.

If there is 1 ball, No measurement is required.
If there are 2-3 balls, we need 1 measurement.[(3^0 +1) to 3^1] If there are 4-9 balls, we need 2 measurement. [(3^1 +1) to 3^2]

Now if there are N balls
and lets say it lies between (3^(x-1) +1) to 3^x
then x = [logN/log3] = Greatest integer of logN to the base 3.
Then minimum no. of measures is equal to x = [logN/log3]

Defective ball

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You have 12 balls all look identical (in shape, color etc.). All of them have same weight except one defective ball. You don’t know that the defective one is heavier or lighter than other balls. You can use a two sided balance system (not the electronic one). It is given that the minimum no. of measures required to separate the defective ball is three. Find the way you separate the defective ball.

See Solution : Defective ball

Divide the balls into 3 groups of 4 balls.

First weigh :
weigh 2 groups, one on each side.
There will be two cases
a) The weight on both side is equal i. e. these two groups don’t have the defective ball.
b) One side has less weight than the other side.
Case (a) :
You know 8 balls are of equal weight and one of the remaining 4 balls have a defective one. Name these four as B1, B2, B3, B4.
Second weigh :
Take B1, B2 and weight them.
(i) If they are unequal then either B1 is defective or B2. Compare B1 with one of eight balls. If B1 is equal to that then B2 is defective otherwise B1.
Total measurements in this case : 1 (first weigh) + 1 (second weigh) + 1 (B1 with other ball) = 3
(ii) If B1 and B2 are equal then either B3 is defective or B4. Compare B3 with one of eight balls. If B3 is equal to that then B4 is defective otherwise B3.
Total measurements in this case : 1 (first weigh) + 1 (second weigh) + 1 (B3 with other ball) = 3
Case (b) :
Mark the balls in the side with less weight as L and with more weight as M. We get 4L and 4M.
Second weigh :
Take 2L and 1M in One side say A and take 2L and 1M in Other side say B of balance system. 2M are reserved for now.(i) If side A is down and next side goes up then it has two possibilities.
1. One of 2L in B has less weight than other 7 balls
2. The 1M in A has more weight(ii) If side B is down then it also has two possibilities.
1. One of 2L in A has less weight than other 7 balls
2. One of 1M in B has more weight than other 7 balls(iii) Both sides are balanced

Third weigh :
In cases (i) or (ii) we will get 3 balls (2L and 1M) after the second weigh.
For case (i) and (ii)of (b) :
Weigh two L balls with each other, if they are equal then the 1M is heavier and if they are not then the ball with less weight is defective.
For case (iii) of (b) :
In this case one of the two reserved balls is defective. We have 2M balls. Weigh them, the one which is heavier is defectives. because we know that issue is with the ball with more weight.

King and wine bottles

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A bad king has a cellar of 1000 bottles of delightful and very expensive wine. A neighboring queen plots to kill the bad king and sends a servant to poison the wine. Fortunately (or say unfortunately) the bad king’s guards catch the servant after he has only poisoned one bottle. Alas, the guards don’t know which bottle but know that the poison is so strong that even if diluted 100,000 times it would still kill the king. Furthermore, it takes one month to have an effect. The bad king decides he will get some of the prisoners in his vast dungeons to drink the wine. Being a clever bad king he knows he needs to murder no more than 10 prisoners – believing he can fob off such a low death rate – and will still be able to drink the rest of the wine (999 bottles) at his anniversary party in 5 weeks time. Explain what is in mind of the king, how will he be able to do so ? (of course he has less then 1000 prisoners in his prisons)

See Solution : King and wine bottles
Hint : Think in terms of binary numbers. (now don’t read the solution without giving a try…)

Number the bottles 1 to 1000 and write the number in binary format.

bottle 1 = 0000000001 (10 digit binary)
bottle 2 = 0000000010
bottle 500 = 0111110100
bottle 1000 = 1111101000

Now take 10 prisoners and number them 1 to 10, now let prisoner 1 take a sip from every bottle that has a 1 in its least significant bit. Let prisoner 10 take a sip from every bottle with a 1 in its most significant bit. etc.

prisoner = 10 9 8 7 6 5 4 3 2 1
bottle 924 = 1 1 1 0 0 1 1 1 0 0
For instance, bottle no. 924 would be sipped by 10,9,8,5,4 and 3. That way if bottle no. 924 was the poisoned one, only those prisoners would die.
After four weeks, line the prisoners up in their bit order and read each living prisoner as a 0 bit and each dead prisoner as a 1 bit. The number that you get is the bottle of wine that was poisoned.
1000 is less than 1024 (2^10). If there were 1024 or more bottles of wine it would take more than 10 prisoners.

Wires on fire

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A guy has two wires of varying thickness, which each burns in 60 minutes. He actually wants to measure 45 mins. How can he measure 45 mins using only these two wires. (he can’t cut the one wire in half because the wires are non-homogeneous and he can’t be sure how long it will burn)

See Solution : Wires on fire

He will burn the first wire at both the ends and the second wire at one end. After half an hour, the first one burns completely and at this point of time, he will burn the other end of the second wire so now it will take 15 mins more to completely burn.. so total time is 30+15 i.e. 45mins.

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