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Do you trust your calculator too much?

By , 7 November 2019

Picture of a child working at a desk with pen and paper and laptop.

Would you notice if your calculator app was lying?
Image: ©iStock.com/South_agency

Imagine you’re at university, doing a maths quiz, with a calculator provided. The problem you’re tasked with reads, “Your grandmother was born in 1942, how old was she in 1994?” You think for a moment and realise it’s a subtraction problem. Into your calculator, you type “1994 – 1942”, and then triumphantly, you record the answer: 60 years*.

Except that’s not the real answer. The calculator lies. If you didn’t spot it, you’re not the only one. Of 120 students who took the same test, only about one in five noticed anything was wrong. At the end of the quiz, only four people said that they thought there was something wrong with the calculator.

Secret science

You might have guessed already that this wasn’t really a maths quiz. Secretly, it was an experiment, run in the United States, to find out more about how people use calculators.

The researchers found two things that made it easier to spot a lying calculator. First, students who were better at maths were also better at finding errors. Second, the calculator’s mistakes were easier to notice if it was a real-world question, rather than just an equation.

Correcting your calculator

There are techniques you can use to help catch common calculator mistakes. Consider the example at the top, 1994 – 1942. By looking at the last digit on each number, you can tell the answer wouldn’t be a nice round number like 60. Real world examples give extra clues too. If you found your grandma was 114 years old, you might want to re-check your answer!

If you’re learning maths right now and want some advice, keep studying. The better you get at maths, the better you’ll get at spotting mistakes. And if you have time, think about what your calculator tells you before you write it down. It probably isn’t lying to you, but you might have pressed the wrong button!

*The real answer? 52. (Update: or as noted in the comments, 51!)

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  1. If one of your grandmothers was born in December 1942 then her age for most of 1994 was 51.

    1. True!
      I’ll see if I can find the precise wording in the paper.

  2. From the paper:

    The question was:
    If your grandmother was born in 1942, how old was she in 1994?

    So yes, there are two answers!

  3. Haha! Nice spotting, Paul. Love it.

  4. I’m confused – can someone help me out? Where/how does the ’60’ result come from? I can only get 52 (or as Paul noted – 51).

    1. Hi Lisa,
      You’re right, the answer is supposed to be 52. But in this experiment, the researchers made the calculator give the wrong answer. To make it hard to spot, the calculator gave the right answers for some questions and the wrong one for others!


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