How to add two-digit numbers (for example 40 + 50).

- Place one number above the other so the tens' place digits and ones' place digits are lined up.
Draw a line under the bottom number.
40
__50__

- Add the two ones' place digits. (0 + 0 = 0).
Place the zero below the line in the ones' place column.
40
__50__
0

- Add the numbers in the tens' place column (4 + 5 = 9) and place the answer below the line and to the left of the ones' place sum
40
__50__
90