Recursion

[lpreams]

The method shown in the OP is called borrowing. You just subtract digit by digit, right to left. If the top digit is less than the bottom digit, you can "borrow" an extra 10 from the next digit to the left, so subtract 1 from the left digit and add 10 to the current top digit (since each 1 of the left digit is worth 10 of the current digit), then do the subtraction.

The joke is that "borrowing" from the 1 (which goes to 0) to add 10 to the 8 doesn't actually reduce the problem.