Why does the order of operations help us arrive at the correct calculation? How does it work, why is it PEMDAS? Why not addition first, then multiplication then groups, or something else?

I took it upon myself to get to the bottom of this question because I realized that I am so familiar with manipulating Algebra and mathematical expressions that I know how to navigate the pitfalls. That instills a sense of conceptual knowledge, but that was a false sense. I did not understand why we followed PEMDAS, until now.

By fully understanding why we follow the order of operations we can gain insight into the way math is written and have better abilities to understand and explain ourselves to others. So, if you’re a teacher, this is powerful because you’ll have a foundation that can be shared with others without them having to trip in all of the holes. If you’re a student, this is a great piece of information because it will empower you to see mathematics more clearly.

Let’s start with the basics, addition and subtraction. First off, subtraction is addition of negative integers. We are taught “take-away,” but that’s not the whole story. Addition and subtraction are the same operation. We do them from left to right as a matter of convention, because we read from left to right.

But what is addition? In order to unpack why the order of operations works we must understand this most basic question. Well, addition, is repeated counting, nothing more. Suppose you have 3 vials of zombie vaccine and someone donates another 6 to your cause. Instead of laying them out and counting from the beginning, we can combine six and three to get the count of nine.

And what is 9? Nine is | | | | | | | | |.

What about multiplication? That’s just skip counting. For example, say you now have four baskets, each with 7 vials of this zombie vaccine. Four groups of seven is twenty-eight. We could lay them all out and count them, we could lay them all out and add them, or we could multiply.

Now suppose someone gave you another 6 vials. To calculate how many vials you had, you could not add the six to one of the baskets, and then multiply because not all of the baskets would have the same amount. When we are multiplying we are skip counting by the same amount, however many times is appropriate.

For example:

4 baskets of 7 vials each

4 × 7 = 28

or

7 + 7 + 7 + 7 = 28

or

[ | | | | | | | ] [ | | | | | | | ] [ | | | | | | | ] [ | | | | | | | ]

Consider the 4 × 7 method of calculation. We are repeatedly counting by 7. If we considered the scenario where we received an additional six vials, it would look like this:

4 × 7 + 6

If you add first, you end up with 4 baskets of 13 vials each, which is not the case. We have four baskets of 7 vials each, plus six more vials.

Addition compacts the counting. Multiplication compacts the addition of same sized groups of things. If you add before you multiply, you are changing the size of the groups, or the number of the groups, when in fact, addition really only changes single pieces that belong in those groups, but not the groups themselves.

Division is multiplication by the reciprocal. In one respect it can be thought of as skip subtracting, going backwards, but that doesn’t change how it fits with respect to the order of operations. It is the same level of compacting counting as multiplication.

Exponents are repeated multiplication, of the same thing!

Consider:

3 + 6

4 × 7 = 7 + 7 + 7 + 7

7^{4} = 7 × 7 × 7 × 7

This is one layer of further complexity. Look at 7 × 7. That is seven trucks each with seven boxes. The next × 7 is like seven baskets per box. The last × 7 is seven vials in each basket.

The 4 × 7 is just four baskets of seven vials.

The 3 + 9 is like having 3 vials and someone giving you six more.

So, let’s look at:

9 + 4 × 7^{4}

Remember that the 7^{4} is seven trucks of seven boxes of seven baskets, each with seven vials! Multiplication is skip counting and we are skip counting repeatedly by 7, four times.

Now, the 4 × 7^{4} means that we have four groups of seven trucks, each with seven boxes, each with seven baskets containing seven vials of zombie vaccine each.

The 9 in the front means we have nine more vials of zombie vaccine. To add the nine to the four first, would increase the number of trucks of vaccine we have!

Now parentheses don’t have a mathematical reason to go first, not any more than why we do math from left to right. It’s convention. We all agree that we group things with the highest priority with parentheses, so we do them first.

(9 + 4) × 7^{4}

The above expression means we have 9 and then four more groups of seven trucks with seven boxes with … and so on.

Exponents are compacted multiplication, but the multiplication is of the same number. The multiplication is compacting the addition. The addition is compacting the counting. Each layer kind of nests or layers groups of like sized things, allowing us to skip over repeated calculations that are the same.

We write mathematics with these operations because it is clean and clear. If we tried to write out 3^{5}, we would have a page-long monstrosity. We can perform this calculation readily, but often write the expression instead of the calculation because it is cleaner.

The order of operations respects level of nesting, or compaction, of exponents, multiplication and addition, as they related to counting individual things. The higher the order of arithmetic we go, up to exponents, the higher the level of compaction.

Exponents are repeated multiplication, and multiplication is repeated addition, and addition is repeated, or skip counting. We group things together with all of these operations, but how that grouping is done must be done in order when perform the calculations.