msu_math

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« on: November 09, 2012, 03:25:20 PM »
Problem:

There are 100 beads on a necklace. 1 of them is red, the rest are blue. Working in the clockwise direction, we start from the red bead and remove every other bead. Right when the red bead is removed, how many blue beads are there left on the necklace?

Solution:

Since initially we have 100 (an even number), on the first cycle we remove 100/2 = 50 blue balls leaving 50 balls with one of them red. The first cycle stopped (the last bead removed in the first cycle) is the blue ball on the left of the red ball. Now the second cycle is like the first only we have now 50 balls and we also begin removing the blue ball right next to the red ball (since we stopped removing the blue ball on the left of the red in the first cycle). Hence we remove 50/2 = 25 blue balls leaving 25 balls with one of them red. Now we have 25 balls and it can be shown that on the third cycle, we finally be able to remove the red bead since we only have an odd number of balls and we also begin removing the bead on the right of red. Hence with the red ball marked as 25 and the ball right next to it as 1, 2,.. ,24, on the third cycle we remove beads 1, 3, 5, …, 25 or a total of 13 beads leaving as with 25 – 13 = 12 blue beads.

« Last Edit: November 14, 2012, 09:28:42 AM by msu_math »

Lecturer in Mathematics
Department of Natural Sciences
FSIT, DIU

bipasha

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Re: A Necklace with 100 Beads
« Reply #1 on: November 12, 2012, 11:15:46 AM »
Very interesting problem and intellectual solution

msu_math

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Re: A Necklace with 100 Beads
« Reply #2 on: November 14, 2012, 09:35:46 AM »
The aforementioned solution can be generalized in case of a necklace with N beads where 1 of them is red and the rest are blue.

Answer:  k -1 blue beads where N = (2k-1)*2q for some integers k>0 and q>=0.
« Last Edit: November 18, 2012, 12:01:15 PM by msu_math »

Lecturer in Mathematics
Department of Natural Sciences
FSIT, DIU

Masuma Parvin

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Re: A Necklace with 100 Beads
« Reply #3 on: November 18, 2012, 11:49:01 AM »
Very interesting!!

shirin.ns

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Re: A Necklace with 100 Beads
« Reply #4 on: March 31, 2014, 11:46:26 AM »
Interesting!!!
Shirin Sultana
Lecturer (Mathematics)
Dept. of General Educational Development (GED)
Daffodil International university

ummekulsum

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Re: A Necklace with 100 Beads
« Reply #5 on: December 02, 2014, 07:33:18 PM »
really interesting one