Bunuel
Josh writes the numbers 1, 2, 3, ..., 99, 100. He marks out 1, skips the next number 2, marks out 3, and continues skipping and marking out the next number to the end of the list. Then he goes back to the start of his list, marks out the first remaining number 2, skips the next number 4, marks out 6, skips 8, marks out 10, and so on to the end. Josh continues in this manner until only one number remains. What is that number?
(A) 13
(B) 32
(C) 56
(D) 64
(E) 96
Are You Up For the Challenge: 700 Level QuestionsQuick answer:
What are we cutting first?
The series starts with 1, and we cut all odd numbers?
What are we left with?
Multiples of 2.
What do we cut next?
The series starts with 2, and we cut Multiples of 2 but not 4
Thus series always starts with 2^n, and is left with multiples of 2^n. Thus the largest positive integer of type 2^n, which is less than 100 is our answer.
64 is 2^6 and our answer as 2^7 > 100.
You can do it step wise too.
I. 1,2,3,4,5…99,100
Cut out odd numbers
II. 2,4,6,…96,98,100
Cut out 2,6,10…, that is we are left with multiples of 4.
III. 4,8,12,…96,100
Cut out 4,12,20…, that is multiples of 4 but not 8
IV. 8,16,24,…96
Cut out 8,24,…, that is multiples of 8 but not 16
V. 16,32,…96
Cut out 16,48,80, that is multiples of 16 but not 32
VI. 32,64,96
Cut out 32 and 96
VII
64
D