Well, if you do it a little, the obvious pattern emerges. You see that either: - There is no need for a carry, and then you just need to remember the previous

Well, if you do it a little, the obvious pattern emerges. You see that either: - There is no need for a carry, and then you just need to remember the previous 10s digit and subtract 7 from the ones place [tiny bit of memory work, single digit subtraction by 7]. - There is a need for a carry, the 10s place goes down by 1, the ones place goes up by 3 [a little extra strain on memory, single digit addition by 3]. Dispatching between those operations is easy. Is the ones place greater than or equal to 7?