Thanks for the comment :)
I know it could have been done with one iteration with reduce, but this way mapping and summing are not wired together. Easier to read and until it does not cause performance issues I prefer these kind of solutions.
I use this pattern to introduce separation of conserns to my students as well :)

Using map and sum iterates 2 times in the String. With a reduce you can subsctract and accumulate.

One thing I learned from this kata: never ever train a kata when you're tired because you will look like an idiot :D

I felt using 15 as cheating so I haven't used it :)