Candidate:

Are we talking about filling stations for vehicles?

Interviewer:

Yes.

Candidate:

Should I take into account CNG stations as well?

Interviewer:

No, just petrol and diesel stations.

Candidate:

The population of the town where I was born and brought up is approximately 12 lakhs.

And it has about 50 – 60 petrol/diesel stations. Let’ go with 55.

This means that each station serves about 22,000 people. (1,200,000 / 55)

The population of India is approximately 1.3 billion.

I will just divide 1.3B with 22K. (1,300,000,000 / 22,000)

This gives me about 60,000 petrol/diesel stations in India. That’s my final answer

Candidate:

Caffeine is found in many foods and beverages like dark chocolate, energy drinks, coffee, tea etc.

Would you like me take all those products into consideration?

Interviewer:

No, let’s just stick to coffee.

Candidate:

Then, I will first estimate the number of cups of coffee consumed in the U.S. last week.

At this point, I would assume that all cups of coffee contain the same amount of coffee; let’s say, the equivalent of a Starbucks Tall cup of coffee which is about 350ml.

May I go ahead with that assumption?

Interviewer:

Yes.

Candidate:

Thank you.

The population of U.S. is about 330 million. Assuming equal distribution of age and assuming that everyone in the U.S. lives until the age of 80, we get 50 million children who fall in the age bracket 0-12. They consume coffee in a very small amount and hence, can be ignored in our calculations.

This leaves us with 280 million out of which I am assuming 20% drink beverages other than coffee. That brings us down to 80%*280M which is approximately 225M.

From my observation and understanding, I can guess that:

50% of 225M à drink 2 cups of coffee per day

25% of 225M à drink 4 cups of coffee per day

25% of 225M à drink 1 cup of coffee per day

Total cups of coffee consumed per day = 50%*225M*2 + 25%*225M*4 + 25%*225M*1

= 550M cups approx.

Fewer cups of coffee would have been consumed on week-ends (Saturday & Sunday), compared to week days (Monday to Friday) when most of the people go to work and feel more need to consume caffeine.

Therefore,

Total cups of coffee consumed in 1 week = 5*550M + 50%*550M*2

= 2750M + 550M

= 3300 million or, 3 billion approx.

Now we are left to calculate the amount of caffeine consumed in the U.S. last week. May I assume at this point that all cups of coffee consumed in the U.S. are cappuccinos which contain a single shot of coffee?

Interviewer:

Sure.

Candidate:

Thank you.

From what I know, an average cup of coffee (350ml) contains about 100mg of caffeine.

Therefore, the amount of caffeine consumed in the U.S. last week would have been:

100mg * 3B = 300B mg.

                      = 0.3 trillion mg.

Candidate:

If I am correct, the 747 in question is the aircraft 747?

Interviewer:

Yes.

Candidate:

I would first like to know whether the aircraft in question is full of passengers?

Interviewer:

Assume there are no passengers.

Candidate:

Ok. How about luggage, food and beverages? I would like to know if the aircraft has any of those things in it?

Interviewer:

No, none of those. The aircraft is empty.

Candidate:

How about the aircraft’s fuel tanks? Are they full, partially-full or empty?

Interviewer:

Consider them empty.

Candidate:

Ok, thank you.

In that case, I would like to break down the aircraft into its major components. After this, I will estimate the weight of each component.

And then finally add up everything to get the total weight of the 747 aircraft.

Interviewer:

I’m happy with that strategy. Go ahead.

Candidate:

Thank you.

Wheels:

An aircraft’s wheels are massive in size. I think a 747 must have about 30 of them.

Interviewer:

It actually has 18 tires.

Candidate:

I see, thank you for the correction.

I have changed a flat tire many times, so I know that a car’s tire weighs around 10 kgs with the rim.

A 747’s tire must be 10 times the size of a car’s tire. So, I think it will weigh around,

10*10 = 100 kgs.

18 tires will therefore weigh,

18*100 = 1800 kgs or,

= 2 tonnes approx.

Wings:

Assuming that the wing is shaped as a right-angled triangle, the area of one wing will be:

= ½ * Base * Perpendicular

= ½ * 25 feet * 200 feet

= 2500 sq. feet

Assuming 2 kgs per square foot gives us:

= 2*2500

= 5000 kgs

= 5 tonnes

For two wings, that comes down to:

= 2*5

= 10 tonnes.

Engines:

An aircraft’s engines are located beneath the wings. Keeping that in mind, I think a big craft like the 747 must have 2 beneath each wing, so a total of 4 engines.

Each must easily weigh about 1000 kgs. Four engines will weigh,

4*1000 = 4000 kgs

= 4 tonnes.

Interior:

Seats: I am assuming that a 747 must have around 500 seats. And my guess is that each seat weighs around 5kgs. So, in all,

= 5*500

= 2500 kgs

= 2.5 tonnes.

Other interior parts include overhead storage compartments, restrooms, light and air conditioning fixtures, cockpit etc.

I will estimate all of this to be around 5 tonnes.

So, the total weight of the interior is,

2.5 + 5 = 7.5 tonnes

Exterior:

The body of a car weighs around 100 kgs. Assuming that the body of a 747 is made up of 50 such frames, the total comes out to be,

= 50*100

= 5000 kgs

= 5 tonnes

Miscellaneous:

For all the miscellaneous parts like carpet, stairs, tail etc., I will add another 3 tonnes.

I have not considered the weight of the air inside the aircraft. Should I calculate it?

Interviewer:

Yes.

Candidate:

Ok. I will add 1 tonne for the air contained inside the cabin.

Adding up all the pieces,

Total = 250 + 2 + 10 + 4 + 7.5 + 5 + 3+ 1

= 280 tonnes approx.

 

Candidate:

That’s an interesting question. May I take a minute to gather my thoughts?

Interviewer:

Sure.

Candidate:

Thank you.

I would first like to know whether you would like me to calculate the number of iphone users in India or would you like me to calculate the number of iphones which are in use as we speak?

Interviewer:

Total number of iphone users in India.

Candidate:

Ok.

In that case I would like to look at the consumption side and use a top-down approach to reach my final estimate.

Interviewer:

Ok, go ahead.

Candidate:

India’s current population is approx. 1.3 billion.

From what I know, the breakdown of the Indian population on the basis of income is as follows:

Poor -> 20%

Lower Middle Class -> 70%

Middle Class -> 5%

Upper Middle Class -> 2%

Rich -> 1%

 

It’s my understanding that it’s only the middle class, upper middle class and the rich who can afford an expensive phone such as an iphone.

I belong to a middle-class family and every 20^th person that I know uses an iphone. So, the number of iphone users in this category should be:

= (5% * 1.3B) / 20

Even though almost every person in the Upper Middle Class and Rich category can definitely afford an iphone, I think only every 15^th person in the Middle-Class category and every 5^th person in the Rich category is an iphone user since there are many other competing brands of smart phones in the market. So, the number of iphone users in these two categories of the Indian population are:

= (2% * 1.3B) / 15 + (1% * 1.3B) / 5

So, the total number of iphone users in India is:

= (5% * 1.3B) / 20 + (2% * 1.3B) / 15 + (1% * 1.3B) / 5

= 0.065B/20 + 0.026B/15 + 0.013B/5

= 0.008B

= 8 million

Interviewer:

Ok, good.

So, will it be correct to say that 8 million people experienced using an iphone in India last year?

Candidate:

That will be incorrect. To calculate the number of people who would have used an iphone in India last year, we will have to take into account the second-hand market of iphones.

Mobile phones frequently change hands in India and therefore, the correct answer will be upwards of 8 million.

Would you like me to calculate the exact figure?

Interviewer:

No, that’ll be all.