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Hannibal1996

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17 Jun 2025, 06:22

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I like the solution - it’s helpful.

PUJABIN

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02 Aug 2025, 08:32

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06 Oct 2025, 03:38

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22 Nov 2025, 05:14

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16 Dec 2025, 21:46

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I don’t quite agree with the solution. need to say you are at the end of the 9th day. Could be day 9.1 still

Bunuel

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16 Dec 2025, 22:48

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Schloop

I don’t quite agree with the solution. need to say you are at the end of the 9th day. Could be day 9.1 still

“Started 9 days ago with 2 live bacteria” fixes the starting point and the number in time. It tells you that the colony had 2 bacteria exactly 9 days earlier. From that moment to now, 9 full daily growth steps have occurred. There is no ambiguity in the timing implied by the wording.

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10 Jan 2026, 16:25

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debitisaccusamus

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25 Feb 2026, 01:16

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odionam

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29 Apr 2026, 05:10

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initial 2 and doubles everyday and 9 days ago means 2*2^10. 9 days ago from today is 10th day today or not, Kindly clear

Bunuel

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29 Apr 2026, 05:20

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odionam

initial 2 and doubles everyday and 9 days ago means 2*2^10. 9 days ago from today is 10th day today or not, Kindly clear

I believe your doubt has already been addressed in the thread. Please review it.

AMaxim

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24 Jun 2026, 00:16

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The explanation is wrong for this question, how is it that tenth day when we are calculating all the generation doubled? We are infact only calculating for the ninth day. the last double multiplication is for the 2 bacterias that are there. If there were 3 bacteria, it would be 3*2^9, proving the explanation wrong.

Bunuel

The population of a bacteria colony doubles every day. If the colony was started 9 days ago with 2 live bacteria and each bacteria has a lifespan of 12 days, how many live bacteria are there in the colony today?

A. 512
B. 768
C. 1024
D. 2048
E. 4096

Bunuel

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24 Jun 2026, 01:43

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AMaxim

Your doubt has already been addressed in the thread. Please review. Hope it helps.

fpavel

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12 Jul 2026, 11:59

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SVaidyaraman

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12 Jul 2026, 17:23

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Key idea: The lifespan is a red herring — the colony started 9 days ago and bacteria live 12 days, so nothing has died yet. Every bacterium ever born is still alive. Once you see that, the problem collapses to plain doubling.

Start: 2 bacteria, 9 days of doubling.

2 × 29 = 2 × 512 = 1024

Answer: C. 1024

Why the lifespan clause exists: it's there to make you attempt a subtraction — compute deaths, or split the colony into cohorts and track which generation expires when. But the colony is only 9 days old and the shortest-lived bacterium survives 12 days, so no cohort has reached expiry. The subtraction is zero.

The transferable move: before building machinery for a complicating condition, check whether it has actually triggered. Same habit applies to tax brackets, depreciation thresholds, piecewise functions, expiry dates — anywhere a rule only activates past a boundary.