Seven Eleven's 100-yen coffee and the case of the extra 1 yen

Seven Eleven’s 100-yen regular coffee: 93 yen plus 7 yen (8%) tax.

Seven Eleven’s coffee is not bad–not to mention cheap and convenient–and I usually go there when I need to get my caffeine fix. I prefer their 120-yen Kilimanjaro blend which I always drink black, but for some reason some stores don’t have it, and even stores which offer it don’t always have it throughout the year.

But the regular coffee is just 100 yen, including the 8% tax. You hand over your 100-yen coin and get your cup. Or pay two 100-yen coins and get your two cups. It’s simple–there’s no change so it’s fast and convenient (it’s even more convenient if you use Eddy card or some other electronic money).

But there’s that one time when I bought three cups and got confused as hell.

Some time ago, just before entering the Ken-Ō Expressway in Hachiōji in Tokyo, we stopped by a store and I went in and bought three cups. I gave the cashier a 1,000-yen bill, got my change and went out.

It wasn’t until I was handing out the cups outside to my friends when I realised there was something not quite right about the change. Because 3 cups was 300 yen, I was supposed to get 500-yen and 100-yen coins in change, nothing else. But what I got instead was a combination of 500-, 100-, 50-, 10-, 5- and 1-yen coins.

Something’s not right.

I handed back the receipt to the cashier (I didn’t even check it first) and asked him to check if it was correct. He took a look at it, told me that yes, it looks right, handed it back, and preceeded to attend to another customer waiting in line.

I’m not one to argue over loose change, so I just threw away the receipt and went outside to drink my coffee. But I was still confused as ever. How did that happen?

I processed the whole thing in my head, between sips of cheap, freshly brewed 7-11 regular coffee.

Maybe it was the caffeine kicking in, because it suddenly dawned on me: 1 cup is 100 yen and 2 cups is 200 yen, that much is true. It flies against logic, but maybe 3 cups is not 300 yen after all!

Now the problem lies in thinking that the price of the coffee is 100 yen. It’s not. The price is 93 yen, and the 100 yen that I always pay is 93 yen plus the 8% tax, rounded down. And when we realise that 93 x 0.08 isn’t a round number, everything suddenly becomes clear.

Seven Eleven regular coffee price table

Cups Unit Price 8% tax Total Rounded down
1 93 x 1 = 93 7.44 100.44 100
2 93 x 2 = 186 14.88 200.88 200
3 93 x 3 = 279 22.32 301.32 301
4 93 x 4 = 372 29.76 401.76 401
5 93 x 5 = 465 37.20 502.20 502

So there you go, 3 cups is not 300 yen after all; it’s 300 plus 1 yen, and that explains the bunch of coins I got from the cashier.

Conventional wisdom says that buying more should cost less per unit, but in this case it’s the opposite: the more you buy, the higher it goes.

So 300 yen is not enough to buy 3 cups of regular 7-11 coffee.

But even that is not quite true. Just buy 3 cups in two orders: buy 2 cups first, pay 200 for that, and buy another cup, for an additional 100 yen. Not as convenient, but you save 1 yen.

Having been thus enlightened, I finished my coffee, entered the expressway, and floored it all the way to Tsukuba.