Suppose there are 100 people standing in a circle. The first person has a gun in hand. What he has to do is shoot the second person and then pass the gun to the third person. Now, the third person kills the fourth person and gives gun to the fifth person.
This process is carried till there is only one person surviving. Can you find out who survives at the end ?
There is a simple logic behind solving this question. If the number of people are a power of 2, then the last person standing will be the person who started it.
Now, here we have 100 people. Thus, we will take the highest power of 2 which is less than the total number which is 64.
100 - 64 = 36.
This means that 36 people are killed.
If you go on with the series of even numbers, the 36 people will end at 72 (2, 4, 6, ..., 72). Thus the gun will be in the hand of 73rd person. Now the remaining people are 64 which is the power of 2. Thus the person who will survive in the end will be the 73rd person since he is starting it.