This happens because when you add two two digit numbers (for example 19 and 14) you first have to add the numbers in the ones place (9+4). Once you get your answer you might have a two digit number (In this case 13) now you place down the digit in the ones place (3) and add the tens digit (1) of the number (13) to the other tens digits (so now you have 1+1+1=3) Place that number down to get your sum(33).