Estimate the market demand in gallons for skim milk consumed in the U.S. in a given year.
Weak Answer
People are more likely to drink milk in the morning, and
kids probably drink the most milk. I’d guess that kids drink 3-4 glasses per
day so let’s say 21 glasses per week. That’s probably like a gallon or so. If
the average family has 1.5 kids, that’s 1.5 gallons per family. But adults use
some milk too for coffee or cereal. That’s maybe another 1/2 gallon per family
so now where are we? I think 2 gallons per family per week. That’s 104 gallons
per family per week times the number of families. I’m guessing 15 million
families with kids so that’s 104 x 15 which is like 15 billion. Oh and there’s
the school consumption which is maybe equal to consumer consumption. The answer
is around 30 billion gallons.
This candidate does not manage the scope of the question. They
focus primarily on consumption of milk by children. The interviewee mentions milk
consumption by adults but only appears to calculate milk consumption by adults who have children. In the end the interviewee brings up schools and haphazardly doubles their
number to get at a final answer. A more structured approach would have helped
considerably.
The interviewee makes assumptions that appear to just be guesses. It’s important to
talk through the assumptions you are making and why so that the interviewer can follow your logic and provide additional information or direct you down a different path if necessary.
This answer also illustrates why simplifying assumptions is
important. The interviewee makes the math overly complicated by starting with glasses of
milk, which need to be converted into gallons. One of their last calculations in
which they multiply 104 times 15 million, is wrong by a large magnitude. When possible,
simplify your assumptions and use round numbers to make the math easier.
Finally, the candidate has forgotten that the question was about
skim milk consumption. The analysis appears to have been for milk consumption
overall.
Strong Answer
The first thing I’d do is define the scope of the
problem. We could be looking at consumer or institutional consumption, for
example. I’ll make an assumption that we are only
considering milk consumed by households, and not milk purchased by institutions
such as schools, restaurants, etc. or milk used to make other products like
cheese. The next thing I’ll do is look at the number of households in the U.S.,
and estimate milk consumption per household. The U.S. population is 300
million. If we assume 3 people per household, then there are 100 mil
households. Households with children probably consume 2 or more gallons per
milk per week, but other households probably consumer well under a gallon since
few adults drink milk as a beverage and if they drink it in their coffee, many
purchase coffee outside of the home. Let’s assume that if we averaged these two
types of households together, the typical household consumes 1 gallon of milk
per week. With 100 million households, there would be 100 million gallons per
week consumed nation-wide. There are many different varieties of milk.
Consumers can purchase whole, 2%, 1%, skim, lactose-free. We’ll make an
assumption that 20% of the market is skim. 20% of 100 million gallons equals 20
million gallons of skim consumed in a week. Multiplying 20 million by 52 gives
us just over 1 billion, 1.04 billion gallons of skim consumed during the year.
The candidate clearly defines the scope of the problem to be solved, milk consumed by households. This makes the question more
manageable, and also demonstrates that there are different paths that can be explored
if prompted by the interviewer. The interviewee talks through the assumptions they are making –
size of the US population and the number of people per household, the market
share for skim milk, and their calculations, making it very easy for the
interviewer to follow the candidate's approach and intervene and provide additional
data where appropriate.
See other examples of strong and weak answers to brain teasers: