if at least one of the numbers has a hundreds-digit then if one of the numbers has a missing hundreds-digit then write a 0 digit in its place. add the two hundreds-digits, and to their sum add the carry digit if there is one. then write the sum of the hundreds-column under the line, also in the hundreds column. if the sum has two digits then write down the last digit of the sum in the hundreds-column and write the carry digit to its left: on the thousands-column.