Lesson: acing estimation questions. In this lesson, you will learn how to ace estimation questions. Google likes to ask estimation questions. Remember how we size the market for Juicero during our first course. These questions ask you to go through a similar process, except your interviewer expects you to come up with your original methodology. Classic example of a Google estimation question is, how many ping pong balls can you fit in a Boeing 747 aircraft? The interviewer is most interested in your ability to efficiently decompose the question into manageable parts and assemble them to arrive at an answer, even if it's off by orders the magnitude. Step 1, cross-examine the interviewer. If I asked you to fit as many ping pong balls onto this airplane as possible, your response would not be to do it, your response would be to ask me why? Specifically, what's the customer persona? What's the customer journey? Is this a business client or a group of users? What are the edge cases? For example, are we creating a ball pit for our passengers to enjoy during a regularly scheduled flight or is this a Guinness Book of World Records attempt where the plane never moves? In which case, can we shove balls into the cockpit, the lavatories, and potentially even glue them on the wings? Can we remove the seats? Assume the interviewer response. I want you to fill the passenger cabin only with ping-pong balls. No putting balls in lavatories, food service cabinets, or removing seats. Step 2, describe your high level strategy for how you will arrive at your answer. Say, I need to know one, the volume of the passenger cabin and two the volume of a ping-pong ball. The answer is the first number divided by the second number. Does that seem reasonable? Step 3, start whiteboarding or screen-sharing two columns, one called assumptions, the second calculations. Start filling in your chart. I can hold a ping-pong ball on my finger, so I'd say it's about one inch radius. What's the formula again for volume, four-thirds times pi times radius to the third power. Then let's call that 4 over 3 times 3 times 1 cubed, which cancels out to 4 inches cubed. Does that sound right? Step 4, notice three tactics I use there. The first was to reference my immediate environment, my finger. The second was to propose a rounded number that's easy to perform mental math on. The third was to ask the interviewer a very pointed question about the formula for volume. By telling the interviewer exactly what I want, I'm not fishing for direction, but still driving for response. Let's continue the answer. Let's use seats to estimate the volume of the cabin. I'm not an aircraft enthusiast, remind me how many columns and rows of seats there are on the 747. The interviewer might say, if there's 50 rows of seats that look like this, so then three seats, then a walkway followed by four seats, then a walkway, then three more seats. Let's assume that walkways take up one seat, so one row consists of 12 seats. The plane consists of 50 times 12 equals 600 seats. What's the volume of one seat? That's similar to the chair I'm sitting in now. About a square with two-foot sides and a height of on average five feet. Because I keep banging my head on the bulkhead. We're not including legroom, but we're also not including the space the actual chair takes up. So I say those cancel out. So passenger compartment volume would be approximately 600 times 2 times 2 times 5 or 600 times 20 or 12,000 cubic feet. Now we need to convert cubic feet to cubic inches. Tell me what is that conversion? So there's 1,728 cubic inches in a cubic foot. So our answer is roughly 12,000 cubic feet times 2,000 cubic inches, over cubic foot divided by 4 cubic inches. Then my answer to the original question is 12,000 cubic feet times 2,000 cubic inches, over cubic feet over 4 cubic inches, or 3,000 times 2,000 or 6 million ping-pong balls. Here are two recommendations for how to get good at an estimation interview question. First, be aware of your surroundings. How many megabytes was that offline Google map, your phone asked to download? How much data does your phone consume per month? How many YouTube videos do you watch per day? How many ads are on each video? Knowing these random facts may help you land that Google job. Second, there's some party games that focus on estimation questions. The one I'm holding right now is called, "Over and Under." It ask questions like, "How many bricks are in the Empire State Building?" Each player makes a brief presentation. Whether playing alone or with friends, these games sharpen your intuition for answering estimation question. Record yourself answering an estimation question.
