Guesstimate Questions: An approach of Evaluation

How much money Gmail makes from ads in every year?

How much money does the shampoo industry earn each year in US?

How many ping pong balls could fit inside of a limousine?

Number of Whatsapp Messages Sent in Mumbai each Day?

Cost of painting the pillars of the metro lines in Delhi?

And many more…

These are sample guesstimate questions, generally a Product Manager or Business Analyst used to face during the interview process.

I promise you: no one cares if you the correct answer. even if you the right answer, it would not help you, it could distract you. This is the once case where the correct answer is not necessarily the best one.

Estimation questions are entirely about the process you take to solve them. Interviewers will use this technique to evaluate your problem-solving skills as well as your quantitative skills.

What’s the relevance to PMing, you might ask? Quite a bit.

Other than the obvious (an ideal PM is good at problem solving and good at math), being able to estimate things is a valuable skill. After all, when you need to figure out what you might be able to expect for the revenue from a given feature, estimation will come in handy. It is the most common way to estimate market size.We just need some tips and tricks to solve these problems.

So let’s start…

We will start with the question that I had faced first time in the final round of interview with the VP of my current Organisation, Mr. Ravi Challu in 2012, July.

The question was: How many ping pong balls could fit inside of a limousine?

When I heard for the first time, I could not understand how to approach the problem.

At first, let me try to understand the problem. You cannot answer a question if you don’t understand it. That’s why it is important to make sure you heard the question correctly and truly understand what being asked. You are free to ask any question to understand the problem, but you should not make any wrong assumption.

So, I asked what is  limousine, as I didn’t know what is that….? And got the answer that a limousine  is a luxury vehicle driven by a chauffeur with a partition between the driver’s compartment and the passenger’s compartment.

I prepared an equation to get the number of ping pong balls= volume of limousine/volume of a ping-pong ball.

So here, I can assume that the length of a limousine is 25 feet and stretch 10 feet with minimum height of 10 feet.

I considered height=10-1.5=8.5 ft, as i left 1.5 ft clearance from the ground.

So volume of limousine= 25*10*8.5=2125 cubic feet.

As it is a luxurious car, i left 50% of it as open space.

So, the total volume within which ping-pong balls will be filled up=2125/2=1062.5 cubic ft

Now, we need to calculate the volume of a ping pong ball.

Radius of a ping pong ball in inches, R=0.75 inches

Volume of a sphere, V=(4/3)pi*r*r*r.

As it is difficult to calculate the r cube, mentally, I assumed r as 0.8 and whose cube is 0.512.

Then V= 2.05 cubic inches.

Now, we must convert the units of the volume of limousine into inches as it will be easy for me to calculate the number of ping pong ball.

I feet=12 inches

I cubic feet=12*12*12=1728 cubic inches.

The volume of limousine in inches=1062.5*1728=1836000 cubic inches.

Now, the expected answer= volume of limousine/volume of ping pong ball=1836000 / 2.05=918000 (approx.)

Tips and Tricks:

  1. Round Numbers: There are times when being a perfectionist and being detail oriented is warranted, but this is not such a time. after all, you are going to be taking so many wild guesses and guesstimates that a bit of hand waving in your math really wont make a difference. In my example, i rounded radius of ping pong ball from 0.75 inches to 0.8 inch to make my calculation bit easier and also considered the value of pi=3 instead of 3.141, so that it will be easy for me to calculate 4/3*pi, which was 4 for me.
  2. Round of 72: Here is a fun. Any value increasing at x% per year will double after 72/x years. Example: currently, an employee makes INR 20,000 per year and his salary increases 9% per year. After how many years, his salary will become double? answer: 72/9=8 yrs.
  3. Label your Unit: To make unit of all measurements universal.  in my above problem, i converted all the units into inches, as i felt it will be easy for me to calculate. you can convert the units to any particular level, as per your convenient, but never forget to do it. Many people just forget to convert, by thinking that it may not have much impact, but there only we fail.
  4. Consider your sources: In many scenarios, we need to consider the sources of information, which will be used in my below problem. In such situations, we should be enough agile to think of all possible sources.
  5. Record Intermediate Steps: While some estimate questions are rarely short but many are fairly lengthy and require numerous calculations. Its important that you write down what you are doing. you might need to come back to it later to correct your work or even to reuse a previously computed number. Record, each step in an organised, easy to read manner, so that you can avoid any wrong calculation.
  6. Record your Assumptions: its not uncommon for your final answer to be wildly off. When this happens, there are two main reasons: either you made a math mistake or one of your assumptions were wrong. Therefore, the easier it is to identify where you made an assumption, the easier it will be to discover potential issues. You should maintain the list of all assumptions, on the lift side of the paper and circle those when those get completed one by one.
  7. Memorize some basic fact: You should memorize some basic facts such as population of the Country, average age of the population, average income of a household and revenue by company: Netflix, Google, Apple, Dropbox etc…

I encountered my second guesstimate question during an interview with MSD, Singapore in March, 2018.

The question was: How much money Gmail makes from ads in every year?

And now, I am giving it to, request you to come up with the approach, make an equation, break the problem and come up with your approximate answer and provide your answer in the comment box.

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