Bacteria Doubles Every Minute: The J.P. Morgan Interview Question

J.P. Morgan interviewers use this question to watch how a candidate thinks, not what a candidate has memorized. Here it is: bacteria doubles every minute, and the colony fills the whole dish in exactly 60 minutes. How full is the dish at minute 59?

The answer: the dish is half full at minute 59, exactly 50%. Because the colony doubles in the final minute, it goes from half full to completely full in that one last step.

Most candidates pause, reach for a percentage calculation, and talk themselves out of the simple answer. This post gives you the full logic, the trap to avoid, the variations interviewers use, and how to answer out loud so the interviewer can follow your reasoning.

Contents

• Bacteria doubles every minute: the answer, worked backwards
• Bacteria doubles every minute: the minute-by-minute table
• The trap behind “bacteria doubles every minute”
• Why J.P. Morgan asks this question
• How to answer it in the interview room
• Variations interviewers use to test you again
• Frequently asked questions
• Key takeaways

Bacteria Doubles Every Minute: The Answer, Worked Backwards

Forward thinking makes this problem feel hard. Backward thinking makes it trivial.

Start at the end. At minute 60, the dish is 100% full. One minute earlier, the colony was about to double one final time to reach that full dish. So at minute 59, the dish held exactly half of the final amount: 50%.

Keep walking backwards and the pattern stays clean. Minute 58: half of a half, so 25%. Minute 57: 12.5%. Every step back divides by two, just as every step forward multiplies by two. One division replaces sixty multiplications, and that is the whole trick: exponential questions are almost always easier in reverse.

Bacteria Doubles Every Minute: The Minute-by-Minute Table

The table shows the last six minutes. Read the final column carefully, because it is where intuition breaks down.

Minute	How full the dish is
55	3.125%
56	6.25%
57	12.5%
58	25%
59	50%
60	100% (full)

At minute 55, the dish looks almost empty at barely 3% full. Five minutes later, it is completely full. That is the signature of exponential growth: nearly all of the visible change happens at the very end. Interviewers love this question precisely because the table shocks people the first time they see it.

The Trap Behind “Bacteria Doubles Every Minute”

The common wrong answer is “about 98% full” or “almost full.” Candidates reach it by thinking in a straight line: if the dish fills in 60 minutes, then each minute must add roughly one sixtieth of the dish, so minute 59 should sit at 59 out of 60.

That logic works for a tap filling a bucket at a constant rate. It fails the moment growth feeds on itself. The phrase “bacteria doubles every minute” tells you the growth each minute is proportional to what already exists, not a fixed amount. A colony that doubles does not add the same number of bacteria each minute; it adds more and more, and the last minute alone adds as much as everything that came before it combined.

When you catch yourself dividing the total time into equal slices on a growth question, stop. Check whether the problem said double, triple, or grow by a percentage. If it did, you are in exponential territory, and linear intuition will betray you.

Why J.P. Morgan Asks This Question

This puzzle shows up in candidate reports from interviews at major investment banks, and it survives because it tests four things in under a minute:

• Exponential thinking. Banking runs on compound growth, interest, and returns. An analyst who thinks linearly about growth will misjudge real problems, not just puzzles.
• Working backwards. Valuation, deal timelines, and error-checking all reward people who can start from the answer and reason in reverse.
• Composure. The question is designed to look like a calculation problem. Candidates who stay calm and reason out loud beat candidates who rush to arithmetic.
• Clarity. The interviewer wants to hear the logic in plain words: “If it doubles and ends full, then one minute before the end it was half.” One clean sentence can carry the whole answer.

How to Answer It in the Interview Room

Say the reasoning out loud, in this order:

1. “The colony doubles every minute, so I will work backwards from the end.”
2. “At minute 60 the dish is completely full.”
3. “One minute before that, it must have been half full, because the final doubling takes it from half to full.”
4. “So at minute 59, the dish is 50% full.”

Then stop. Do not decorate the answer with extra calculations. If the interviewer pushes, extend the same logic: minute 58 is 25%, minute 57 is 12.5%. Showing that you can continue the pattern without hesitation confirms the first answer was reasoning, not a guess.

More puzzles in this format: work through the 8 balls puzzle asked at Goldman Sachs and the 4 cars puzzle next, both from the same interview series on this site.

Variations Interviewers Use to Test You Again

Interviewers rotate this puzzle so candidates cannot survive on a memorized answer. The two classic variations:

The lily pad version. Lily pads double in area every day and cover the whole pond in 48 days. On which day is the pond half covered? Day 47, by the identical logic: one day before full coverage, coverage was half. This version appears in the Cognitive Reflection Test created by psychologist Shane Frederick in 2005, a test built around questions where the first answer that comes to mind is usually wrong.

The tripling version. A colony triples every minute and fills the dish in 60 minutes. At minute 59, the dish is one third full, because the final step multiplies by three instead of two. Candidates who memorized “half” without understanding the logic fail this one immediately, which is exactly why interviewers ask it.

Frequently Asked Questions

What is the answer to the bacteria colony interview question?
The dish is half full at minute 59. The colony doubles every minute, so one minute before the dish is full at minute 60, it holds exactly half of the final amount: 50%.

Why is the answer not 59/60 or about 98%?
Because the colony grows by doubling, not by adding a fixed amount each minute. Equal slices of time do not mean equal slices of growth. The final minute adds as much new growth as all 59 previous minutes combined.

What if the colony triples every minute instead?
Then at minute 59 the dish is one third full. Work backwards and divide by the growth factor: one step before full is full divided by three.

Is this a real J.P. Morgan interview question?
Candidates interviewing at major investment banks, J.P. Morgan among them, have reported this puzzle and its variations in interview rounds for years. It belongs to a family of exponential-growth brain teasers that banks use to test reasoning under pressure.

Key Takeaways

• When something doubles and ends full at minute 60, it was half full at minute 59. Work backwards and divide.
• Exponential growth hides at the start and explodes at the end: barely 3% full at minute 55, completely full at minute 60.
• The linear answer, 59 out of 60, is the trap. Growth that feeds on itself never fills in equal slices.
• In the interview, state the backward logic in one clean sentence, give the answer, and stop talking.

Preparing for bank interviews? Save this puzzle, then test yourself on the variations above without looking at the answers. If you can solve the tripling version cold, you understand the idea well enough for any interviewer.