after the second number has been written down under the first one so that digits line up in their correct columns, draw a line under the second (bottom) number. start with the ones-column: the ones-column should contain a pair of digits: the ones-digit of the first number and, under it, the ones-digit of the second number. find the sum of these two digits: write this sum under the line and in the ones-column. if the sum has two digits, then write down only the ones-digit of the sum. write the "carry digit" above the top digit of the next column: in this case the next column is the tens-column, so write a 1 above the tens-digit of the first number.