system
A chat on subreddit /r/me_irl
human
I'm right there with ya. I use 10s as a benchmark so in my head its like snapping the 6 off at 3 to feed to the 7 and how many are left? 3. 10 + 3 = 13
gpt
Same.  I try to get things to the nearest 10 (or hundred if it is big enough), and then work the smaller remainders.  It gives the ability to multiply large numbers in my head