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Brain Teasers 2

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bucknast

bucknast

Mechanical
Apr 10, 2012
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I've got an interview with a company tomorrow that's notorious for asking difficult brain teasers.

Do you have any such questions that you remember from any of your interview experiences?
 
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By the way, I answered the coffee teaser with:

For the hottest coffee thirty minutes from now, mix them whenever you want and nuke it in the microwave for a minute when you're ready to sip.

The thermo calculations can't be done anyway, there are too many unknowns.

I got a big smile from the interviewer. It might have been a sarcastic smile, though. I didn't get the job offer.

Best to you,

Goober Dave

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Heat loss (from the coffee/cream system) is primarily proportional to the temperature difference from the ambient environment. Mixing the two immediately will lower the rate of heat loss and result in a higher temperature compared to mixing later. The hot coffee is at a greater difference in temperature from ambient than the chilled cream so there is no advantage to waiting so the cream can warm. If you get hung-up on not knowing enough details, then perhaps you are not the type of engineer that is being sought.
 
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[dadgum arthritis -- poor mousemanship sometimes means a too-quick "submit post"]

If the coffee's in a heated pot, all the heat it loses will be replaced during the half hour. Wait and pour it then.

Run some tap water or hot water over the cream container in the breakroom sink. Give it some extra heat to start with.

Etc...

In other words, you're right, Compositepro, in one sense. If he were looking for an engineer who knows his or her stuff, and knows when to stop digging for data, your answer gets the job.

I would personally prefer to hire the engineer who suggests methods that would improve the outcome over the two less-than-optimal methods he was given to evaluate. I would also prefer to work for someone who expects questioning.[glasses]

Best to you,

Goober Dave

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Coffee question:

I'll make (and state*) for the interviewer some environmental assumptions: We have no external heat inputs available, there are two containers (one for cream, one for coffee, and that we have significantly more coffee than cream. The coffee will be significantly above ambient temperature, the cream will be moderately below ambient temperature. These are the "best fit" of likely conditions - coffee around 190C, cream around 2C, environment around 20C.

Immediately add the cream to the coffee, then place the cream container on top of the coffee cup to arrest evaporative cooling and reduce convective cooling.

Ha!

*Throw in a question back, such as "Okay, is that the limit of the data I have on hand or can reasonably obtain?" to make sure it isn't one of those "lets see how you research" type of questions.
 
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fegenbush: Clearly you have never experienced the sort of coffee they make in my office.

"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." -Scott Adams
 
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That remindes me of my one trip to Spain.
Got to the hotel breakfast buffet, and poured a cup of coffee and a touch of hot milk.
The coffee was terrible, but I needed a second cup regardless.
This time I was behind one of the natives, and noticed he poured about 1/4 cup and the rest milk. I followed suit, and behold, a latte! The reason for the hot milk.
 
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My favourite brain teaser is the one where you are sitting in a boat in a small pond, holding a brick. Your throw it overboard. What happens to the water level in the pond?

- Steve
 
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That's pretty much what I told the aggressive, smug interviewer who asked me that question. He said I was wrong and then gave a wrong answer himself (claimed that for the water level to stay the same, the brick would need a relative density of exactly 1).

I was 17, hundreds of miles from home, desperately needing to find a sponsor and quite intimidated by the panel of 3 interviewers. So I didn't argue the case. Or get the job.

- Steve
 
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Trick question. You don't sit and dawdle and daydream with the refrigerator door open. What are you doing, trying to air condition the great outdoors?
 
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Check the wattage label on the refrigerator, measure the volume of the room, figure how many calories you eat in a normal day & convert to watts, add together for total heat released to room in joules, use the specific heat of air (just for simplicity's sake) and the volume of air for temperature increase over time. (Presumably, the air inside the refrigerator counts in the initial estimate of the room's temperature.)

Unless you mean the cooling rate of the room, in which case you'd need to measure the efficacy of the insulation and the difference between the inside/outside temperature.

"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." -Scott Adams
 
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