Girl math is (sort of) legit, according to this economics expert

It's a trend that started on social media -- women make fun of themselves for the "crazy" mathematics they do when it comes to spending and saving.

But there are some aspects of girl math that are actually real concepts, according to Concordia University economics professor Moshe Lander.

"It's a way to try and convey, across social media, something that's probably taught in a classroom that's never absorbed," he muses.

He breaks down the economic legitimacy of some girl math declarations:

Claim: If I buy an item for $100 and wear it 10 times, it only cost me $10 per wear.

Lander: "That is correct. That's called the average cost.

Let's say that it costs you a billion dollars to build an auto factory.

What you want to do is build as many cars in that factory as possible so when you spread the billion dollars over each of the cars you've produced, that's the average cost of the factory embedded within the cars.

Same logic: if you're going to buy an article of clothing, if you're going to buy an iPhone, then you want to get the most usage out of it as possible to reduce the average cost per use."

Claim: If I put items in my online cart but don't hit buy, I made money.

Lander: (Laughs) "No, I don't think that's math in any way. That's just not spending.

Unless they get some sort of endorphins from the actual shopping experience, they didn't actually get anything.

So there's no savings there to be had. There's no making money."

Claim: If I return an item for $90 and then go out and spend $100, I really only spent $10.

Lander: "At the margin, that is correct, but it's only at the margin.

There's a difference between thinking about the total of something and the margin of something.

Let's say that you and I are in a relationship, and we've had 10 terrible years together but tomorrow's a really good day, does that mean that the relationship has been great? No.

They had to spend that $90 before to be able to return it.

So, in total, they spent $190, and they got back $90 on the item that they returned, so they still spent, in total, $100."

Claim: If something I want is on sale or buy one get one free, I not only save money, I basically make money.

Lander: "Let's keep it simple: If you have a buy one get one free promotion, you still spent money, it's just that instead of spending money on one item, you spent that same amount of money and got two."

Claim: If I spend an extra $30 to get free shipping, I get more bang for my buck.

Lander: "Maybe...that might be true. It depends on what the shipping cost is because even though you're spending more money in total, you're getting more goods and services with it.

It depends, though, on what those extra goods and services you're getting in relation to the shipping cost."

Claim: If I underspend on my budget this week, I can overspend next week.

Lander: "I'll give you a nonfinancial analogy.

If you're on a diet and you're trying to lose weight, do you say to yourself, 'Well, I didn't use all my calories this week, so I can pig out next week.'

You can't say, 'well, I can just have this gigantic piece of sugary cake because I didn't have a lot of sugar during the day.'

It's more justifying to yourself than it is good budgeting."

Claim: If I pay cash, it doesn't count because there's no proof of spending.

Lander: "No. That's 100 per cent not true. Spending is spending whether you have a receipt for it or not, whether you pay cash under the table or whether you pay with an electronic transfer.

No, you spent the money."

Claim: If something like toilet paper is 30 per cent off and I buy a year's supply, then I saved 30 per cent.

Lander: "If you're capable of estimating your toilet paper needs for the year, then yes, you can save 30 per cent if you buy it on sale.

That's bargain shopping. That's good strategy."

Claim: If I spend $100 once a month for five months instead of spending $500 in one shot, it's good budgeting.

Lander: (Laughs). "No, $500 is $500. That is what I would call childish thinking.

'Can I have my next four weeks allowance today? I promise I won't ask for money for the rest of the month,' and then next week, they're asking for money again."

Lander says he wonders if girl math has gained popularity because young people almost exclusively use digital transactions.

"Gen Z really was the first generation that really didn't have to have money if they didn't want it," he said. "The sort of 'scan' that they're running on is essentially a way of trying to move digital bits and bytes around."

He notes if people were still predominantely using hard cash, most of these examples of girl math -- or perhaps, Gen Z math, would disappear.

"You'd realize that at the end of the day, when the cash is gone, the cash is gone. It doesn't matter how you got rid of it," Lander states. "Because most people live in an electronic world, they're not familiar with the idea of the feeling of getting cash."

-- Answers have been shortened for clarity.