Google brain teaser interview questions: the famous ones, answered

Try the six classics

Four have a single right answer; two are estimates, where landing within the reference range counts as correct. Each comes with the reasoning an interviewer wanted to hear.

The six classics, answered

Each answer, where the question comes from, and what it was really testing.

1. 1 · Google, reportedYou are shrunk to the height of a nickel and thrown into a blender. The blades start moving in 60 seconds. What do you do?

   Answer

   Jump out. When you shrink, your mass falls much faster than your muscle strength, so like a flea you can jump many times your height.

   How to get there

   1. 1Notice the question is about scale, not survival tricks.
   2. 2Mass falls with the cube of height, muscle strength roughly with the square.
   3. 3So a tiny person is far stronger for their weight.
   4. 4That makes jumping clear of the jar plausible, which is the answer interviewers hoped for.

   Mastery Point: When a setup is absurd, look for the physical or economic principle it is hiding.

   What it was testing

   Whether you look for the principle behind a strange setup instead of improvising escapes.

   Where it comes from

   Reported as a Google interview question in the 2000s, and discussed in William Poundstone's book Are You Smart Enough to Work at Google? (2012).

2. 2 · Google, reportedHow many golf balls fit in a school bus?

   Answer

   About 500,000 to 700,000.

   How to get there

   1. 1Inside of a bus: about 2.5 m wide, 2 m high and 10 m long, so 50 cubic meters.
   2. 2Take off about 10% for seats: 45 cubic meters, or 45 million cubic centimeters.
   3. 3A golf ball is 4.3 cm across, so about 42 cubic centimeters.
   4. 4Randomly packed spheres fill about 64% of a space.
   5. 5Result: 45,000,000 x 0.64 / 42 is about 690,000.

   Mastery Point: State the packing assumption out loud; it changes the answer by a third.

   What it was testing

   Breaking a volume problem into dimensions, and remembering that spheres leave gaps.

   Where it comes from

   Widely reported as a Google interview question; a staple of interview-prep books since the 2000s.

3. 3 · Classic Fermi problemHow many piano tuners are there in Chicago?

   Answer

   About 50 to 100.

   How to get there

   1. 1Chicago has about 3 million people, about 2.5 per household, so 1.2 million households.
   2. 2Say 1 household in 20 has a piano: 60,000 pianos.
   3. 3A piano is tuned about once a year: 60,000 tunings a year.
   4. 4A tuner does about 4 a day, 250 days a year: 1,000 tunings each.
   5. 5Result: 60,000 / 1,000 = about 60 tuners.

   Mastery Point: Estimate demand and the capacity of one worker, then divide.

   What it was testing

   Building a number from demand and supply, with each assumption said out loud.

   Where it comes from

   The classic Fermi problem, associated with the physicist Enrico Fermi, who set his students questions like it. It became a staple of consulting and tech interviews.

4. 4 · Classic tech puzzleHow many times a day do the hour and minute hands of a clock overlap?

   Answer

   22 times.

   How to get there

   1. 1The minute hand gains one full lap on the hour hand each time they meet.
   2. 2In 12 hours the minute hand goes round 12 times and the hour hand once, so it gains 11 laps.
   3. 3That is 11 overlaps every 12 hours.
   4. 4Result: 22 in a day. The obvious answer, 24, forgets that the hands do not meet between 11 and 1 except at 12.

   Mastery Point: Count relative motion, not each hand on its own.

   What it was testing

   Whether you check the intuitive answer instead of saying it.

   Where it comes from

   A classic puzzle in tech and quant interviews.

5. 5 · Microsoft, 1990sWhy are manhole covers round?

   Answer

   A round cover cannot fall through its own hole, whichever way it is turned. It can also be rolled, and it needs no lining up to be put back.

   How to get there

   1. 1Ask what a cover must never do: fall into the hole and onto a worker.
   2. 2A square cover can slip through along its diagonal.
   3. 3A circle has the same width in every direction, so it cannot.
   4. 4Add the practical extras: it rolls, and it fits back at any angle.

   Mastery Point: Start from the failure the design must prevent.

   What it was testing

   Reasoning from what a design must prevent, then adding secondary benefits.

   Where it comes from

   Made famous by Microsoft interviews in the 1990s, then asked across the tech industry.

6. 6 · Lateral-thinking riddleA man pushes his car to a hotel and tells the owner he is bankrupt. Why?

   Answer

   He is playing Monopoly: his token is the car, and he landed on a property with a hotel he cannot pay for.

   How to get there

   1. 1Each detail sounds normal but none of them fits a real story: you do not push a car to a hotel to announce bankruptcy.
   2. 2Ask where a car, a hotel and bankruptcy all belong.
   3. 3A board game with a car token, hotels and a player going broke.

   Mastery Point: When every detail is odd, change the frame rather than the details.

   What it was testing

   Switching frames when a literal reading does not hold.

   Where it comes from

   A classic lateral-thinking riddle, common in puzzle books and early tech interviews.