This calculation seems very simple, almost childish. Yet, it poses a challenge for many adults, because we tend to go too fast and forget the order of operations learned at school.
At first glance, it’s a calculation you could give to a child. A few numbers, plus and minus signs, a multiplication… nothing complicated. And yet, this simple exercise trips up a great many adults. Why? Because we all tend to rush, calculating in the order we read, without thinking about the rules we learned at school. And that’s precisely where the trap lies.
The problem that trips everyone up
Here is the calculation:
15 – 3 × 4 + 6 = ?
Take a few seconds to do it mentally, without a calculator, as if you were in class. Many people find 54, 48, or even 18… when the result is something else entirely. It’s not a problem with the calculation itself, but a problem with the method.
In mathematics, there is a very important rule that is often forgotten over time: the order of operations .
The order of operations: the rule never to forget
When there are several operations in the same calculation, you don’t calculate from left to right. You must follow a specific order:
Multiplication and division;
addition and subtraction
This means that multiplication must be performed before addition or subtraction, even if it is in the middle of the calculation.
It is this rule that completely changes the result.
Let’s solve the calculation step by step
Let’s go over the calculation again:
15 – 3 × 4 + 6
The first step is to perform the multiplication:
3 × 4 = 12
The calculation then becomes:
15 – 12 + 6
Only now can we calculate from left to right:
15 – 12 = 3
3 + 6 = 9
The final result is therefore 9.
It wasn’t that complicated, but you still had to remember to do the multiplication first.
Why many people are mistaken
See more on the next page
