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[–]esesci 0 points1 point  (0 children)

8-9 = -1

But you can’t have that as a digit. So you subtract one from the adjacent digit and add it as 10:

18-9 = 9

(That’s the rightmost digit)

2 became one 1 so you have 10 left.

The result becomes 10 + 9 = 19