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Brainteaser: Three Pills

(Image credit: Flickr user Isabel Odriozola)

If a doctor gave you three pills and told you to take one every half hour, how long would they last you?

Continue reading for the answer.

One hour.


This brainteaser is reprinted with permission from Uncle John's Supremely Satisfying Bathroom Reader.

Since 1988, the Bathroom Reader Institute had published a series of popular books containing irresistible bits of trivia and obscure yet fascinating facts. If you like Neatorama, you'll love the Bathroom Reader Institute's books - go ahead and check 'em out!


Do you take one now, or in a half-hour?
Does 'last' mean when they are gone, or when you'd normally take another?
If you take one now, then one every 30min, they're gone in an hour... but your next pill would be due in another 30min.
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If you take one right away, then the other two on schedule, the pills will be ingested in the course of an hour, but the effects may last longer. E.g.: #1 at 12:00pm, #2 at 12:30pm, and #3 at 1:00pm. Done. (So the next time you feel the need to deride your friendly pharmacy technician based on her pharmacy math skills used to calculate how long your medicine should last, well, think again. You are welcome.)
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Poorly phrased question. If this is something critical to your livelyhood, you care about when you need more by, not when you gobble the last one. In that case, they "last" an hour and a half, not an hour. If you care about the impact of the medicine on your ability to perform other tasks, like driving or take other medicine which may have interactions, then they will "last" even longer than an hour and a half until it safe.

The question should be at least closer to the subject, like "when will you run out of pills?"
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The type of error one commits when answering a question like this is sometimes called a 'fencepost error' because one has to be careful to distinguish between the upright fence posts and the rails between them. I have a nearly useless blog post about fencepost errors, frames and motion blur (useful mainly for starting arguments between animators on a Friday night.)
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confusing question, these are the answers I thought of before looking

1 hour 30 min, take one 30 minutes after the doc gives you the pills

1 hour

30 minutes, each pills effect lasts 30 min

or 2 hours and 30 min, stupid person thinks you can only take them on half hours
example: got pills at 2:00 took first one at 2:30 next at 3:30 and last at 4:30
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