Its call the complement of 9. It is how the old time mechnical adding machines worked, numbers 0-9 were on a wheel and when you revesed directions you subtracted by adding the complement of 9.
357 357
- 162 + 837 The complement of 9, what number added to 1 gives you 9, =8, what is the complemnet of 3 then 7
-------------------------
1 94 8+3=11 the 1 in the 100 collumn and add 1 to the units
+1
_________________
195 answer
