The Monkey Trainer

The director of Hind Circus has decided to add a new performance called the monkey dance to his show. The monkey dance is danced simultaneously by N monkeys. There are N circles drawn on the ground. There are N arrows drawn between the circles in such a way that for each circle, exactly one arrow begins at that circle and exactly one arrow ends at that circle. No arrow can both begin and end at the same circle. When the show begins, each monkey sits on a different circle. At each whistle of the ringmaster, all the monkeys simultaneously jump from one circle to the next, following the arrow leading out of the current circle. This is one step of the dance. The dance ends when all the monkeys have simultaneously returned to the circles where they initially started. The director wishes the dance to last as many steps as possible. This can be achieved by drawing the arrows intelligently.

For N =15 the maximum number of steps that the monkey dance can be made to last by drawing arrows appropriately?


The answer is 105.

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3 solutions

Priyanka Banerjee
Dec 24, 2013

Nice problem we see that for the largest circle the dance completes in 15 steps and so we need more than 1 independent circle. After some calculation till 4 different circles we can show that the largest possible number of steps will be when there are three circles of 3, 5 and 7 monkeys and their time period will be LCM of the numbers

Answer 105

nice one... :)

Surya Teja Cheedella - 7 years, 5 months ago
Adit Mohan
Dec 25, 2013

we can arrange the circles in groups.
The number of steps taken for all monkeys of a group to return = number of circles in group.
let number of circle (or monkeys) in groups = a,b,c...
no of total steps = LCM(a,b,c..).
also a+b+c..=15.
clearly the acceptable values for a,b,c.. which allow maximum LCM are 3,5 and 7.
thus answer = 105



I dont understand this problem at all. If you had say N circles how could you draw N arrows to get any more than N steps?

Nahom Yemane - 7 years, 5 months ago

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see, you don't connect them all together in a n-gon what you do is you connect them in smaller groups such that when one group's monkeys are back in their original position the other groups' are not the maximum possible is by forming groups of 3,5,7

Adit Mohan - 7 years, 5 months ago

They made 3 rings of circles : 1 ring of 3 circles + 1 ring of 5 circles around the first one + 1 ring of 7 circles around the first two.
Total number of circles / arrows
= 3 + 5 + 7
= 15.
Total number of moves
= 3 x 5 x 7
= 105.

Saya Suka - 7 months, 3 weeks ago

Say we numbered the monkeys from 1 to 15, with 1 ~ 3 in the first group dancing the innermost circles, 4 ~ 8 in the second group dancing the in-between circles and 9 ~ 15 in the third group dancing the outermost circles. All the circles will also be numbered the same as the monkey that occupied each one at the start of the dance (before the first step / jump). So after the first 3 steps, the first group will have returned to their original circles, but the same cannot be said for the other groups. M4 will be occupying C7, M7 at C5 after 3 jumps (originally at C7, moving on to C8, C4 then C5), M11 at C14, M15 at C11 etc. So for all of them to finish simultaneously, inner monkeys have to make 35 smallest rounds, in-between monkeys doing 21 complete midsize rounds of 5 and the largest group to revisit their original positions 15 times before they're done with the dance.

Saya Suka - 7 months, 3 weeks ago

clearly the acceptable values for a,b,c..

How is it 'CLEAR' to you? Can you explain it a bit more in detail?

Abhishek Sharma - 7 years, 3 months ago

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They made 3 rings of circles: 1 ring of 3 circles + 1 ring of 5 circles around the first one + 1 ring of 7 circles around the first two.
Total number of circles / arrows
= 3 + 5 + 7
= 15.
Total number of moves
= 3 x 5 x 7
= 105.

Saya Suka - 7 months, 3 weeks ago

Say we numbered the monkeys from 1 to 15, with 1 ~ 3 in the first group dancing the innermost circles, 4 ~ 8 in the second group dancing the in-between circles and 9 ~ 15 in the third group dancing the outermost circles. All the circles will also be numbered the same as the monkey that occupied each one at the start of the dance (before the first step / jump). So after the first 3 steps, the first group will have returned to their original circles, but the same cannot be said for the other groups. M4 will be occupying C7, M7 at C5 after 3 jumps (originally at C7, moving on to C8, C4 then C5), M11 at C14, M15 at C11 etc. So for all of them to finish simultaneously, inner monkeys have to make 35 smallest rounds, in-between monkeys doing 21 complete midsize rounds of 5 and the largest group to revisit their original positions 15 times before they're done with the dance.

Saya Suka - 7 months, 3 weeks ago
Brian Chen
Dec 24, 2013

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