Adding two three digit numbers (for example 129 + 733) involves several steps.
Place one number above the other so that the hundreds, tens and ones places are lined up. Draw a line under the bottom number.
Add the ones place digits (9 + 3 = 12). This number is larger than 10 so place a one above the tens place column and place the two below the line in the ones place column.
Add the tens place digits (1 + 2 + 3 = 6) and place the answer below the line and in the tens place column.
Add the numbers in the hundreds place column (1 + 7 = 8) and place the 8 below the line.
Learn to explore 3 – digit addition with and without regrouping the ones below.