Use subtraction:
987 - 89:
Line up our numbers vertically:
| 9 | 8 | 7 | |
| _ | 8 | 9 |
| 17 | |||
| 9 | 8 | ||
| _ | 8 | 9 | 8 |
Column 1:
7 - 9 is < 0, we need to borrow 10.
Column 1: 10 + 7 - 9 = 8
| 17 | 17 | ||
| 9 | |||
| _ | 8 | 9 | 9 | 8 |
Column 2: 8 - 8 is <= 0, and we needed to borrow on the last column.
Therefore, we need to subtract 1 for the borrow in the previous column, and then add 10 for another borrow for this column. Total borrowing is 10 - 1 = 9.
Column 2: 9 + 8 - 8 = 9
| 8 | 17 | 17 | |
| _ | 8 | 9 | 8 | 9 | 8 |
Column 3:
Since we borrowed on the last column, we take it out of 9
Column 3: 9 - 1 - 0 = 8
Final Answer
Answer = 987 - 89 = 898
