Human calculators typically train for extreme speed in certain categories of written mathematical operations, such as square roots, flash addition and prime factorization. But today, we’re thinking about how speed mental math can be useful—or amusing—in everyday situations.
Whether you’re a beginner training mental math, or a world record-holder in calculation, it would be fun to hear from you if you have used your skills in a useful situation, or to surprise people in everyday life.
Maybe you have helped someone with your number abilities, or you’ve ended up in a funny situation because of being faster than a calculator in real life. Please share in the comments if so! Below are 6 of mine:
📅 Human Calendars in Wetzlar
At the World Cup in 2012, held in Gießen, many of us went on a trip to the nearby historic town of Wetzlar for a guided tour. At one point the guide showed us a plaque commemorating a historical date, like 27th July 1748.
Many of us had just been competing in the Calendar Dates event, including some of the fastest in the world like Yusnier Viera, Jan van Koningsveld and Nodas Boukovalas. So the guide had a surprise when several of the group immediately shouted “that was a Saturday!”
🏃♂️ Human Fitbit
There’s a great 5000-meter running track around Margitsziget in Budapest, with regular distance markings. So while I was staying nearby I went there, aiming to break my record for running 5 kilometers. By combining my basic stopwatch and mental calculation, I could regularly check my running progress. 1100 meters in 4:45? Too fast and I’ll burn out! 3700 meters in 17:00? Need to hurry up to beat my time!
An added bonus of this is that calculating various statistics, such as speed in miles per hour, was a welcome distraction while running the course.
🍛 The Restaurant Bill Scam
I once went to a restaurant with some friends, ordering lots of food and drink. Before the bill came, I’d quickly calculated the total and found the official bill was significantly higher! So we checked the items in detail and discovered that the restaurant had added some duplicate items onto our receipt.
It turned out that this restaurant was deliberately doing this to scam the customers into paying more, knowing that most people would be too slow with numbers to check. We asked them to “correct” our bill, and never went back.
🌎 Foreign Tourist Traps
On the same theme of scams, ATMs and hotels in foreign countries sometimes give really bad exchange rates when travelling. The worst I’ve seen was a Euronet ATM in Budapest, which tried to add on 13% extra! If you know the conversion rate—or a two-step rule to make it easier—you can quickly check whether the rate is acceptable.
This was especially helpful in Costa Rica, as the exchange rate to USD was an awkward number (about 560) and several places asked me to pay in dollars but with a poor conversion rate.
🎂 Guess My Age
Have you ever wondered someone’s age but didn’t like to ask? If you can do day-of-week calculations fast enough, you can offer to demonstrate! Most people will be skeptical that you can do it, and unless they want to hide their age—fair enough if so—they’ll typically tell you their date of birth and ask you for the day of week they were born.
“18th June 1818? That was a Thursday! But are you sure you’re 200 years old…?”
🌋 Volcán Telica
While hiking in Nicaragua, we came to the active volcano of Telica, which looks like your perfect textbook image of a volcano. There’s a huge drop from the edge into the lava and we were curious how deep it was. Luckily there was a way of solving this problem. We threw a rock horizontally from where we were stood and timed the number of seconds for it to reach the bottom. Then using the suvat equation:
\(distance = 0.5 \times 9.8 \times time^2\)
…we could estimate the depth of the crater. If the rock was falling for 4.9 seconds, how deep was the volcano?
