Fish with binary speed

There are n n fish numbered 1 1 through n n at one end of an aquarium. They all start swimming at the same time to the other end of the aquarium. Once a fish reaches an end of the aquarium, it turns around immediately and continues swimming in the other direction.

Fish 1 1 swims with speed v 1 v_1 , fish 2 2 swims with speed v 2 = 2 v 1 v_2 = 2 v_1 , fish 3 3 swims with speed v 3 = 2 v 2 v_3 = 2 v_2 , and so on.

Will the n n fish all be at the same end of the aquarium at the same time again?

An example aquarium with \(n=3\) fish. An example aquarium with n = 3 n=3 fish.

Yes No It depends on the value of v 1 v_1 It depends on whether n n is odd or even

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12 solutions

Zee Ell
Jan 17, 2018

It is easy to see, that while the first (slowest) fish swims one round (back and forth), the second fish swims two rounds, the third fish 4 rounds ... and the nth fish 2 n 1 2^{n - 1} rounds.

Hence, all fish meets at the starting point whenever the first fish completes a round.

Therefore, our answer should be:

Yes \boxed { \text { Yes } }

The answer must be no. The tank has 2 ends and the Q does not mention which end it just says the same end. When Fish 1 is at the right end Fish 2 will be at the left.

Andrew Christofi - 3 years, 4 months ago

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When fish 1 makes it to the second end fish 2 will have completed a cycle. When fish 1 makes it back to the first end fish 2 will have completed another cycle. Same relationship between 2 and 3, 3 and 4, etc.

Reginald Reed - 3 years, 4 months ago

how does zeno's paradox resolve this?

Δ π - 3 years, 4 months ago

Nice. I was misled by the thought that the ratio v_1 : length of the aquarium had to be rational for this to happen.

Tony Solomonides - 3 years, 4 months ago

The question does NOT specify they stop at any point and if you take 10, it's almost impossible to match the location of 10 fish to be at the end of one aquarium, however long it takes.

The ANSWER is STUPID!!! Or the question is misappropriated!

Imtiaz Hasham - 3 years, 3 months ago
Raisingh Mandloi
Jan 29, 2018

There would be a least common multiple of n quality.

Morten Vindberg
Jan 29, 2018

fish n \text{fish}_{n} will have a speed of 2 n v 1 2^{n} \cdot v_{1} , therefore we know that all fish has a common factor of speed, which (I think) implies that they will eventually meet up at the same end.

2^(n-1) Times v1

Zaher Ilbih - 3 years, 4 months ago

Yes i got the same result as Zaher. But I think your reasoning about having a common factor of speed is correct.

Jesse Boldt - 3 years, 4 months ago
Jonah Hamilton
Jan 29, 2018

If you think of the distance of the pool as a unit then v1 travels 0.5 in the time v2 travels 1 and v3 travels 2. A fish will be at the right hand side of the pool on an odd number of units traveled and on the left side when an even amount. So if all fish can be at either an even or odd amount at the same time then it is possible.

Since each fish is twice the last, any time v1 is even the others are even. Ex: V1= 2 so V2=4 and V3=8

Kumar Raj
Jan 31, 2018

Their meeting time at starting point will be lowest common multiple of their individual time of covering a round.

Matt Buck
Jan 30, 2018

One can deduce the answer without doing any maths. Clearly the answer cannot be "it depends if n is of our even" because if it works for n+1 fish that same value will work for n fish. Similarly it cannot depend on v because v is arbitrary and can only every be expressed in terms of tanks per time unit, in which case we just adjust our time units. Finally we are left with yes or no. Since it clearly works for n=2, the answer must be yes.

Glenn Craig
Jan 30, 2018

The slowest fish had gone half way when the other two had gone 1, and 2 laps. By the time the slowest fis has completed 2 laps, the other two wil have completed 4, and 8 laps respectively

Yash Ghaghada
Jan 29, 2018

(v1=v)=>(v2=2v)=>(v3=>4v).....

let at time t they are at same place again

for (fish one t=(2^a)x/v),(fish two t=(2^b)x/2v),...... (x=length between two ends)(a,b,.. are independent of each other)

for same time the so a=1,b=2,c=4,....

so yes its possible

Ankit K
Feb 4, 2018

It is easy to see that when 1st fish swims 1 round then 2nd fish swims 2 round, 3rd swims 4 rounds, 4th swims 8 rounds and so on. Hence at a particular all fishes will be at same end.

Matthew French
Feb 2, 2018

It's like rolling through the various different combinations on a combination lock but doubling the amount of twists you perform on each digit.

4 Digit Lock 0000, 1248, 2486, 3624, 4862, 5000, 6248, 7486, 8624, 9862, 0000 10 Moves back to zero

7 Digit Lock 0000000, 1248624, 2486248, 3624862, 4862486, 5000000, 6248624, 7486248, 8624862, 9862486, 0000000 10 Moves back to zero

Matteo Marconi
Jan 30, 2018

They will be at the same end when the n-th fish has done 2^n one way trip, the n-1 th 2^(n-1) ... and the first (the slowest) 2 one way trip

Fish 1 1 swims with speed v 1 v_1

Fish 2 2 swims with speed v 2 = 2 v 1 v_2 = 2 v_1 ,

Fish 3 3 swims with speed v 3 = 2 v 2 = 4 v 1 v_3 = 2 v_2 = 4 v_1 ,

Fish 4 4 swims with speed v 4 = 2 v 3 = 8 v 1 v_4 = 2 v_3 = 8 v_1 ,

Fish n n swims with speed v n = 2 v ( n 1 ) = 2 n v 1 v_n = 2 v_(n-1) = 2n v_1 ,

\therefore yes as 2 v 1 , 4 v 1 , 8 v 1 , 16 v 1 . . . . . 2 n v 1 v 1 2v_1, 4 v_1, 8 v_1, 16 v_1..... 2n v_1 || v_1

v n = 2 n 1 v_n = 2^{n-1}

Ossama Ismail - 3 years, 4 months ago

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