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Especially relevant to University of Oxford and University of Cambridge.
Say your answer out loud first, then jot down the key points you made. Writing it helps you spot gaps — and it's what the AI feedback reads.
0 words
Estimation, comfort with numbers, stating assumptions and sanity-checking — without a calculator.
A structure to build your own answer on — not a script.
Clarify
An average person; an order-of-magnitude estimate is fine.
Assume
Average heart rate ~70 bpm; lifespan ~80 years.
Calculate aloud
Per hour → per day → per year → lifetime; round as you go.
Check
Higher in childhood and exercise, lower in sleep — do these roughly cancel? Is the order of magnitude sensible?
Extend
Identify the most sensitive assumption; consider other species.
A strong but not perfect answer. Read the notes to see why it works — then make your own version with your experiences.
I'll aim for an order-of-magnitude estimate.
I'll assume an average heart rate of around 701 beats per minute and a lifespan of about 80 years.
70 beats a minute times 60 minutes is 4,200 beats an hour. Times 24 hours is roughly 100,000 beats a day — 4,200 times 24 is about 101,000, so 100,000 is a nice round number2. Times 365 days is about 36 or 37 million beats a year. Times 80 years is roughly 3 billion.
To check it: my heart rate is higher as a baby — well over 100 — and during exercise, but lower during sleep, perhaps 50 or 60. Those probably partly cancel out3, and even if my average is off by 10 or 20%, the answer is still 'a few billion'.
The assumption with the biggest effect is probably lifespan, although heart rate and lifespan both scale the answer proportionally.
For a mouse, the heart rate is much faster — hundreds of beats a minute — but the lifespan is only two or three years. I've read that many mammals4 end up with a similar number of heartbeats in a lifetime, around a billion, which is a curious pattern — humans seem to be an outlier on the high side.
Real interviews are won and lost here. Answer each one aloud.