While everyone knows that doing physical exercise is incredibly important for staying healthy as you age, many people don’t think about the importance of mental exercise.

According to scientific studies, doing puzzles that stretch and challenge your brain can be vital in keeping your mind sharp and fighting the effects of aging.

Today, we have a math challenge for you. Most people have pretty dismal memories of math from school, and besides, who needs to do math in their minds these days when a calculator is as close as your smartphone? But the benefits of doing math have nothing to do with a machine, so do it in your head instead!

So don’t cheat. Do it the old-fashioned way!

Here’s today’s problem. Pretty simple, right? Go ahead, let’s see your solution. It probably looks something like this.

Step 1: 50 + 50 = 100. Easy as pie.

Step 2: 100 – 25 = 75. Super simple.

Step 3: 75 x 0 = 0. Hold on, wait a minute. All that, for zero?

Step 4: 2 + 2 = 4. As simple as it gets!

If your answer is 4, then you’ve missed it. Your school math teacher would be circling it with an angry-looking red pen. Where did you go wrong? It all seemed so straightforward?

Let’s go back and see what went wrong. If you got the answer above, the problem wasn’t any of the simple arithmetic you did to get the solution. But hold on, if all the individual math was correct, then what else could it be?

Here’s a hint. It’s not what you did; it’s the order you did it in!

You might or might not remember from middle school that as problems started to become longer and more complex, you got some very specific rules about what to do first, next, and last in a problem. These rules are called the “order of operations.”

In a problem like the one above, you have three different kinds of operations: addition, subtraction, and multiplication. But which of the three comes first?

First, remember that you always, always work from left to right. You might have learned a nifty mnemonic device for this in school, like “**P**lease **E**xcuse **M**y **D**ear **A**unt **S**ally.” Starting to sound familiar?

This stands for Parentheses, Exponents, Multiply, Divide, Add, Subtract. Some people have a slightly different version that goes “**G**eorge, **E**xcuse **M**y **D**ear **A**unt **S**ally,” which substitutes “grouping” for parentheses. Regardless of how you remember it, this rule is vital for solving today’s problem. So let’s give it another shot!

Coming back to our problem, 50 + 50 – 25 x 0 + 2 + 2, we can see that the multiplication in the middle needs to come before we do anything else. So let’s rewrite the problem to show this: 50 + 50 – (25 x 0) + 2 + 2. We know that anything multiplied by zero is zero.

Now, we can just add and subtract from left to right. First we calculate 100 – 0 = 100, then 2 + 2 = 4. So the grand total is … 104.

Pretty amazing how a small detail can change everything, isn’t it? So test the problem out on your friends and family, making sure that they don’t try to whip out their calculators to solve it. These days, most calculators already have the order of operations programmed into them, so it would be a cinch.

But where your brain is concerned, it’s definitely a case of “no pain, no gain.”