Yeah, yeah, you can do it normally and just figure out that fifteen minus seven is eight. But you can also take the tens-complement to seven (which is three) and add that to the last digit which is five and that makes eight. It works for all single-digit numbers subtracted from a number 10–19. When you’re subtracting from an even bigger number, it’s not great (subtract ten rather than just lop off the first digit. So 123 minus 7 is absolutely not 6, it’s 116).
Maybe this is just a me-problem it’s just my weird brain (because I was out of school in second grade and missed out on subtraction drills) but I hate subtraction. It’s easier if I convert it to 7+X = 15, my brain right away returns with “eight” when seeing 7+X = 15 whereas when it sees 15 - 7 it returns with grumbling and rusty creaking and paaainful struggle of “okay first subtract the fives so we have two below ten that is eight” or I dunno what the heck it’s doing back there but it’s not fun for it (🧠) to do. But if I’m gonna juggle the numbers to turn it to 7+X = 15 anyway I might as well juggle them to be 3+5 instead because that’s even easier.
I’ve talked about this before but in D&D you often have to add the same static number over and over again to freshly rolled numbers off a twenty-sided die and then compare it to another static number. Let’s say you’re fighting a giant crab with AC 15 and you have a dinky li’l weapon that lets you attack with +7 on your twenty sided die rolls. You roll a 15. You add seven. That is 22. You hit. Then you roll six. You add seven. That is 13. You missed. Then you roll seven. You add seven. That is 14. You missed.
Compared to if you could just know that all rolls eight or higher are hits; you roll 15, that’s a hit. You roll six, that’s a miss. You roll seven, that’s another miss. That’s three additions you didn’t even have to make. That means the math is undeniably easier. There’s a whole step that’s elided!
Even if it’s just on a single roll I do like it when all the math is pre-figured and the roll immediately tells you the hit. You are fighting an AC 13 skeleton with that +7 attack.
In the early years of game design where fiddly li’l things like dice protocols were all we cared about partly because systems weren’t open source back then so everyone had to reinvent the wheel and partly because we hadn’t looked up to see the larger structural issues around prep and role GM/player role separation, this was call “fortune at the end”. Some hated it because they want to keep on fiddling with the dice for all time so it doesn’t see so random what’s happening. You might have rolled the six but then you add your skill of seven afterwards so that it’s your own skill that pushed it over the edge. (And the Fortune-at-the-end haters love it even more when there’s a resource like the compost in Three Sisters or the aspects in Fate that lets you actually change the results.) But I love fortune at the end and am always advocating for it. First place your bets, find your footing, notch your bow, lift your sword, and then let it fall.
Over the years I’ve tried encapsulating this in house rules. Back in 2014 for that first few months of D&D I made a box of SRD monsters on A7-sized cards where their attack values were pre-calculated to be tens complement so a skeleton’s +4 scimitar was listed as 6 and I had noted all the player character’s AC minus ten. So if ten skeletons attacked a character I could just look at that character’s noted number, add six, and grab ten 20s and roll them and quickly pick up the hits. That was all on the DM side and the players didn’t have to worry, for them it was the good old 3e system working as per ushe. This was only for a few years before we introduced “defense rolls” and those were normal D20+adds.
For our last campaign this spring I tried pushing the Suldokar’s Wake-style rules on them instead. As in, if you have a +7 sword attack vs an AC 15 crab, you hit on 7 or lower or 15 or higher. Exact same amount of hits and misses (out of the twenty sides, there are thirteen hits and seven misses) but it’s not the exact same specific ordinal numbers that hit. I.e. a natural 13 is a hit with D20+adds but a miss in the “roll extremes” system. That was mega confusing to the players and they hated it.
So I’m gonna bring back D20 + adds. Vanilla same-as-every-other-5e-table D20 system. I know, I know, the players have pledged in the past to subtract before they start rolling but they never do. Subjective time is one part of this, but maybe there is some other appeal for them in adding after the roll and I’m the outlier. Whatever. That’s fine. I guess the main takeaway here for me is to let the players do their job whatever way they want to do it, and their job is to hit or miss. I don’t want to micromanage that. DMs don’t know what players want.