Elizabeth is adding tens: three tens, four more tens. Find the missing numbers: three tens plus four tens is the missing number, and 30 plus 40 equals the missing number.
Let’s count how many tens there are altogether: one, two, three, four, five, six, seven. Three tens plus four tens is seven tens. Now we need to add 30 and 40. Let’s count in tens to find out how many tens there are altogether: 10, 20, 30, 40, 50, 60, 70. 30 plus 40 is 70. Three tens plus four tens is seven tens. 30 add 40 is 70.