Use subtraction:
109 - 63:
Line up our numbers vertically:
1 | 0 | 9 | |
_ | 6 | 3 |
1 | 0 | 9 | |
_ | 6 | 3 | 6 |
Column 1: 9 - 3 = 6
10 | |||
1 | 9 | ||
_ | 6 | 3 | 4 | 6 |
Column 2:
0 - 6 is < 0, we need to borrow 10.
Column 2: 10 + 0 - 6 = 4
0 | 10 | ||
9 | |||
_ | 6 | 3 | 0 | 4 | 6 |
Column 3:
Since we borrowed on the last column, we take it out of 1
Column 3: 1 - 1 - 0 = 0