Let's talk about Amazon Prime and Walmart+.

Amazon has an estimated 180 million Prime members, paying as much as $139 per year. My family has a membership; chances are you do, too. Launched in 2005, I couldn't even tell you how long I've been a Prime member, but it's certainly been more than a decade.

Walmart was much later to the game, only announcing its Walmart+ program in 2020, and reportedly growing to about 59 million members by last year. I did not think I was one of them. But recently, I realized that as a result of being an American Express card member, I had a membership for free.

That personal revelation came courtesy of a very grownup conversation at a pool party that prompted me to check my credit card benefits.

And it also came just before I saw that Walmart said it's been closing an important gap between its paid membership product and Amazon's (not surprising given Amazon's 15-year head start), which has to do with order frequency and size.

In short, it's comparably easy to get someone to buy something from you once (not easy, but comparably easy). It's usually more difficult to get them to buy things from you repeatedly, and even habitually.

That challenge seems to be the target that Walmart went after in its competition with Amazon, and it's a genius move. (Pun intended, but Target isn't really a part of this conversation yet, given that its paid membership program only just launched in April.)

Speaking at the Evercore ISI Consumer & Retail Conference, Walmart CFO John David Rainey disclosed a few key milestones in its quest for repeat business:

First, the sheer increase and growth Walmart has seen in eCommerce, versus in-store sales: "The eCommerce opportunity for us is growing at 20 percent year-on-year consistent. We're just seeing a lot more customers that are coming to us in a manner that they haven't historically, where maybe historically we were more of a brick-and-mortar business."

Second, how often Walmart+ members make purchases: "What's great about Walmart+ is the behaviors that we see of those customers. Notably, they spend about twice as much as an average customer. ... [Also], they tend to shop more frequently and also have smaller baskets. They're using us for those one or two items."

Third, the growth comes at the expense of other competitors: "I think that speaks to how convenience is resonating with customers. Also, sometimes that maybe they historically would have thought about using another retailer before."

Interestingly, Amazon is mentioned only once by name in the entire roughly 6,600-word transcript (well, once in a question and then once again in Rainey's answer).

But I think we all know who most people assume Rainey refers to when he talks about customers who might have "us[ed] another retailer" besides Walmart.

(I asked both Amazon and Walmart for comment on this story, but I haven't heard back from either company.)

Besides the fact that I'll probably try Walmart+ for the heck of it, since I now realize I've had a membership for some time without even knowing, it's interesting to see how Walmart addresses the overall membership gap.

Walmart+ has a lot of members, but it's still many fewer than Amazon. However, as Rainey put it:

"I'll say this, our success is not going to be solely determined by the number of memberships that we have. It's a nice complement to the overall opportunity."

I might be more skeptical of this explanation if it weren't for the extra focus on order volume and order size.

But in that context it makes sense, and I think it's a great lesson for anyone operating in any industry.

So many business problems are really just math problems. At the least, there are math problems mixed in with everything else.

If you don't understand the formula you're working on--for example in the Walmart+ case, whether you should care most about number of members, average number of purchases, or cart size--it's almost impossible to find the right answer.

So, apply this to your business.

Can you improve the metrics you judge your business by, and that you use to compare your business with others? Are you actually looking at the right numbers?

Maybe you're doing better than you think you are. Maybe you're not--but it's better to know.

Oh, and check your credit card membership benefits. You might find you've got some valuable rewards coming that you had no idea about.