When you add 7 and 8 it equals 15 so if you were to add 17 and 18 it would become 35 because that extra ten from the 7 and 8 can equal a ten which is moved over to the tens place. Like if you did 12+29 you do the ones. then the last number from the 11 (1) goes in the ones and you move the first one over do the tens which equals 3 + 1 in the tens so it equals 41.
