Amoeba spliting

Algebra Level 2

Given that in a room there is an amoeba (only one) and it duplicates itself every second and will fill the entire room in 1 hour. When (in seconds) will the amoebas fill up half the room?


The answer is 3599.

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12 solutions

Zhaochen Xie
Apr 16, 2015

the last second it doubled from a half to full room therefore 3600-1=3599

HOW IS THIS POSSIBLE? I AM NOT ABLE TO UNDERSTAND

Prakhar Gupta - 6 years, 1 month ago

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It is given that the amoeba makes himself double in 1 second. Given that the amoeba will fill the room in 1 hour, the previous second the room would be half filled and the next second every amoeba will double itself and fill the room.

Dhruv Tyagi - 6 years ago

At 1800 seconds the room would be .25% full then wouldn't it be half full at 1801

Tim McHale - 6 years, 1 month ago
Mike Gerber
Apr 20, 2015

This question should be worded differently. According to the statement only one amoeba is doubling.

Siddharth Singh
Apr 17, 2015

Can amoeba fill the remaining half in just a second, that is half the room and also why is amoeba referred to as he? :)

I chuckled to the reference of amoeba as "he" :)

Abyoso Hapsoro - 6 years, 1 month ago

Don't think too seriously. it's just a question :)

Sultan Fatahillah - 6 years, 1 month ago

Because it doubled from half the room to full room

Jordan Knebel - 6 years, 1 month ago

The room will fill in 3600 sec( 1 hour),if it doubles for every second the room filled at 3599 sec is half of the room,at 3598 sec is quarter of the room,at 3597 sec is half if the quarter of the room and so on.......

Sai Prasad - 6 years ago

It's like a tap filling a bottle - each new drop sticks to the other drops and pushes the water level up. Multiplying cells divide locally right next to each other so they're pushing out to fill the room at each division. Imagine that at the last second, the density of the blob doubles (since a second ago before the doubling, the colony would have been 50% as sparse). In reality, cells are dying as well so you'd have to account for cell count with a little more realistic model.

Manjunath Sreedaran - 6 years, 1 month ago

Cause it is an Amheba

Dreamy Delinquent - 5 years, 1 month ago
Raymond Tat
Apr 20, 2015

Just for fun:

Mass of amoeba 1 0 8 grams \text{Mass of amoeba} \approx 10 ^ {-8} \text{grams} Volume of amoeba 1 0 8 grams / water density = 1 0 8 cm 3 \text{Volume of amoeba} \approx 10 ^ {-8} \text{grams}\ / \text{water density} = 10^{-8} \text{cm}^3

Volume after 3600 seconds \text{Volume after 3600 seconds} 1 0 8 cm 3 × 2 3600 \approx 10^{-8} \text{cm}^3 \times 2^{3600}

1 0 989 × Volume of Universe \approx 10 ^ {989} \times \text{Volume of Universe}

Also, some time during the third minute, the amoebae will collapse into a black hole.

Rahul Singh
Apr 18, 2015

Let N(t) be bacteria count at any time 't' According to the question,we have N(t)=2^t. Now when the room is full ,t=3600 & N=2^3600,Where N is the bacteria count when the room is full.Now when the room is half filled, n(t)=N/2 or, 2^t=(2^3600)/2 On solving we get t=3599. It is quite fascinating 2 observe that Half of the remaining bacteria multiplies in just a second. That's the power of exponential increment!!

http://en.wikipedia.org/wiki/Wheat and chessboard_problem

Manjunath Sreedaran - 6 years, 1 month ago
Christy Wong
Apr 18, 2015

From the information the question gives us, we know that:

  • it doubles every second
  • after one second the amoeba fills half the room, it doubles so it would fill the whole room.
  • it would fill the room in 1 hour.

Therefore the room would be half full 1 second before 1 hour, which is 3600 seconds, so the room would be full after 3600-1=3599 seconds. :)

Owen Leong
Aug 11, 2015

Every second the number of amoeba doubles, that is there

is 1 amoeba at the 0th second

2 amoeba at the 1st second

4 amoeba at the 2nd second

and so on.

At the xth second, there are 2^x amoeba

Let's say in 1 hour there are 2^n amoeba

then half of the room would be (2^n)/2 amoeba and

this would be simply the second before an hour.

Le Thanh
Aug 2, 2015

As amoeba duplicates itself every second, we would have: 1 second = 2 amoebas; 2 seconds = 4 amoebas; 3 seconds = 8 amoebas; so.....we would have 2^n amoebas at n seconds. Within 1 hour = 3600 seconds, we would have the amoeba filled full the room with 2^3600 amoebas, suppose t is the time the amoebas​ fill up half the room, we would have 2^3600 = 2*2^t. Solving this equation, we would have t = 3599.

Chloe Tiara
Apr 23, 2015

The time it would take for the amoeba to fill up the room is 3600 seconds. This is obtained by multiplying 60 seconds by 60 minutes (there are 60 seconds in a minute and 60 minutes in an hour). Knowing that the amoeba self-double each second, we can state that half a room was filled in 3599 seconds, one second less than 3600. (Except if only 1 amoeba had split. It is possible with how the problem is worded. This would mean that it will take double the time, which would be 7199 seconds.)

Zi yang Lim
Apr 19, 2015

All amoebas replicate themselves each second. If in 3600 seconds the room was filled, it means that the previous second contained half the amount of amoebas. 3600 - 1 = 3599

Marcos Gonzalez
Apr 19, 2015

Démosle la vuelta al enunciado. 2^3600 amebas llenan una habitación. Si el número de amebas se divide por 2 cada segundo, ¿en qué segundo habrá la mitad de amebas? La respuesta es obvia.

Faizan Khan
Apr 17, 2015

3600-1=3599

Moderator note:

Why? Why not 3600 - 2, 3600 - 3, or others?

Because it become double in one second so the time is needed to fill the half of the room from the beginning is 3599 second . you need to read the problem carefully :)

Refaat M. Sayed - 6 years, 1 month ago

I've already posted a detailed solution..check it out..!!

Rahul Singh - 6 years, 1 month ago

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