Let Us consider 3 digits number, say 721 the result will be:

• Copy the 2 digits at the extreme right and left, 71
• Make space between both of them, 7 1
• To the left of the extreme right digit (of the result) put the addition of the first and second digit (1+2=3), 7 31
• To the left of the second (from the right) digit of the result, put the addition of the second and third digit (7+2=9), 7931
• Please note that when you adding any 2 digits if the result is greater than 10, put only the right digit and carry the left one (the tens) to the one at its left

Another Example 732:

• 7 2
• 7 52
• 8052 ( notice when we added 3+7=10, we put the 0 and we carried the 1 to the 7 which became an 8)

Another example, 356:

• 3 6
• 3 16
• 3916