To draw up a bill

Why write out an addition problem?

As soon as a calculation becomes too difficult to do in your head (multi-digit numbers), you write out the addition in columns. The principle is this: we write the numbers one below the other, aligned on the right, then add them digit by digit, starting with the ones.

The method, column by column

We align the ones under the ones, the tens under the tens, the hundreds under the hundreds, and so on. Add from right to left. If a column exceeds 9, write down the units digit of the result and carry 1 over to the next column.

Simple example: 245 + 128

• Units: 5 + 8 = 13 -> write 3, carry 1
• Tens: 4 + 2 + 1 (carry) = 7 -> write 7
• Hundreds: 2 + 1 = 3 -> write 3

Result: 245 + 128 = 373

Example with two carry-overs: 327 + 486

• Units: 7 + 6 = 13 -> write 3, carry 1
• Tens: 2 + 8 + 1 = 11 -> write 1, carry 1
• Hundreds: 3 + 4 + 1 = 8 -> write 8

Answer: 327 + 486 = 813

Pitfalls to avoid

• Misaligning the columns, especially when the two numbers have different numbers of digits (always align from the right, never from the left).
• Forgetting to carry the carry over to the next column.
• Adding from left to right instead of starting with the ones: this makes it easy to lose track of the carry-overs.